in theory, if they control the mining network, they could freeze/censor transactions they believe are linked to capital flight. it's quite a paranoid theory but it makes slightly more sense than just outright 51% attacking the whole network or something. there's no point in that, as hash rate would just leave china and the compromised pools.
That's noticeable as well. If you want to effectively censor a transaction(s), you need to be able to mine every single block since other mining pools would probably be incentivised to mine those transactions if they aren't mined. It'll be apparent once the other mining pool realizes that they cannot mine any blocks and the same thing will happen. It'll be far more effectively to just execute a massive doublespend attack and massively devaluate Bitcoin at one go, at least they stand to gain something from that.