It may not have been the question but you can be sure it will have been a major factor in it's rejection.

Remember the national moto of France, Liberté, égalité, fraternité

With respect, you reference the old motto; by the early '90s (in Haute-Savoie at least) the motto had morphed into Liberté, égalité, choucroute

True, the deterrent probably isn't as strong in the EU as it is in other countries, where much harsher penalties (including jail time) can apply.

Plus I can still see a lot of people just being "verbally cautioned" when exiting through a Spanish airport...

Yes, very noble, although I'm not sure it's always been rigorously adhered to either inside or outside of France.

And as you yourself point out, there are legal distinctions between the treatment of EU nationals and non-EU, so presumably no domestic barrier exists to prevent distinctions between some non-EU nationals and others. Maybe some already exist?

Of course there are distinctions, but these are result of negotiation and careful consideration of what's going to be fair and acceptable to everybody, not just spur of the moment tweaks to the rules based on what someone thinks might be a nice idea.
EU citizens who are exercising FoM by living in another member state from their own, have certain rights and protections under EU law. All member states have agreed to grant these rights to EU citizens living in their country, and have enshrined them in national law.
So in France for instance, EU (non French) citizens living in France have rights under French law but also rights and protections under the FoM Directive as EU citizens. French citizens living in France get their rights under French law, not EU law, so sometimes a Froggy and his Spanish neighbour will not have exactly the same rights. (Eg there are different rules for bringing a TCN partner to France depending on whether you are French or other EU - under EU rules you don't need to be married or PACS'd, you just have to prove a stable relationship to have the right to be joined by your partner. Whereas French citizens under French law only have the absolute right to be joined by a married partner; the prefecture can use its discretion as to whether to allow them to be joined by a PACS'd partner; and if you're neither married nor PACS'd then no chance even if you've lived together for donkeys' years.) Then there are various bilateral treaties between France and other countries that give favourable treatment in certain specific areas, eg Algerians have a fast track to residency and I believe fewer conditions to meet; some countries such as Canada and Australia have working holiday visa agreements., hopefully one day the UK will get round to negotiating a WHV but so far they haven't. Where there are no special privileges arising from a signed agreement,, bilateral or otherwise, TCN from whatever country are all treated equally under national law.

I would have thought Spain operated pretty much the same, it's how most countries operate.

Thanks for the detailed response ET.

Should put to rest the recurring theme that non EU nationals are / must be treated identically.

I take your point about bilateral agreements, careful consideration etc vs spontaneous tweaks. However, my point was from a purely legal angle - there wouldn't, to the best of my knowledge, be anything legally preventing parliament from passing such a law and it standing.

No, it wasn't taken into consideration, they were looking purely at whether the proper Senat process had been followed or not. As I recall, the statement declaring the proposal rejected on the grounds of process, specifically stated the ruling had been made without prejudice to any other issues.
If it hadn't been chucked out at the first hurdle, then the question of whether it was discrimnatory would have arisen at the second hurdle. FWIW most people felt its chances of getting past the second hurdle were very slim indeed. Then if it did get through, there was still a chance that it would have joined the many other bits of legislation that in theory have been passed, but in practice never actually get brought in.
Not saying it will never happen but I don't think this was seen by most politicians as a serious issue. The big issues are how soon should immigrants be allowed access to benefits and healthcare, how tough should France be on sans papiers, what level of integration is required to try and prop up social cohesion. The plight of Brit homeowners never really came into it.
The anti Macron paper La Libération has had a lot to say about how many amendments and what a disaster it will be for France, but I don't think it had a single mention of the Brit homeowners proposal being dropped.

EES is a fully automated system and the penalties will be automatic and logged onto the system against said person.

The only way there could be any discretion involved is if someone went to a manual booth (in the same way that third country EU residents will for example) and pleaded their case ie they were unavoidable delayed with a connecting flight or they were too ill to travel.

Spain decided to treat Spanish citizens the same as non-Spanish EU citizens when it comes to family reunification rights, i.e. non-EU family members of Spanish citizens apply for an EU family member card just as non-EU family members of e.g. French citizens living in Spain would.

What can letally prevent a law being passed is that France has a written constitution, and no French law can contradict that constitution.
The Conseil Constitutionnel rejected as unconstitutional more than 30 items from this proposed new immigration law passed by the Sénat, not including the home owners thing which never even made it to that stage. So it's not like the Conseil Constitutionnel usually rubber stamp things and it's rare for anything to get crossed out, although usually of course the Sénat doesn't propose laws that it knows aren't constitutional.
I don't know what the constitution says about discrimination but whatever form this proposal comes back in, it would need to be carefully designed not cross any red lines.
But then, as a result of these 30+ articles being judged unconstitutional, there are now calls for the constitution to be overhauled on the grounds that times have changed and France's needs now are different from when the constitution was written. And there is also debate on where foreign policy ends and immigration law begins - the Sénat has its say on immigration law but does not have a say on foreign policy and the international treaties France signs eg with Algeria, and politicians being politicians they don't like that.
So it really isn't a simple matter, and practically, I think that once everbody has finished shouting about this law and it's all been hacked out and implemented, anything to do with immigration and visas will be off the menu for a good while because France will have its a bellyful of immigration. Time to move on to the economy or whatever's next..

That's interesting, thanks, and it seems to me a simpler way to do it. But France is too cocky by half, I suppose it would feel it had lost face if it let the EU tell it how to treat its own citizens.

French citizens living in France could follow the Surinder Singh route as UK citizens living in the UK used to before Brexit to show they've used their EU Treaty Rights by living for a time in another EU country before returning.