I didn't say it was nice, just that it was possible. I mean, what's your alternative explanation for what happened? Murdering them without revoking citizenship doesn't make it better.
Yeah, I think I'll side with the "they didn't pass the Soong test". Otherwise, this is one NASTY plot hole. I'd believe a lot of things about the Federation, but, wholesale, public genocide seems a bit much.
Laws can be changed and in any event Data wouldn't really count in terms of human rights.
Citizenship in the modern era can also be revoked for a variety of reasons.
This isn't citizenship. It was determined, in a Federation court, that Data was a lifeform. This isn't a law and it cannot be revoked. And, again, I'm not really ready to believe that the writers would be that blatant about making the Federation into genocidal mass murderers. Maybe I'm wrong, but, this would be a HUGE right turn for the Federation if I am. Prime Directive violation and Nazi level evil.
Now, maybe the "Soogn Test" isn't quite as... good as it could be and these synthetics were branded as non-sentient when they should not have been. Or, there is a conspiracy to doctor the tests in order to destroy the synthetics. But, I have a serious problem with the notion that the Federation would go back on its own principles to that degree, publicly and in full view of everyone and commit widespread genocide.
Was there any evidence in the show that the 3000 synthetics on Mars were the only synthetics in the Federation? Seems a bit weird if ALL the synthetics suddenly revolted, gathered in one location, and all got rounded up.