But, it can't know that I'm voting or just spending my money, unless they forbid me from using other forks, which doesn't make sense because chains are independent and don't interact. Centralized blockchains (which is a contradictory term anyway) could be coded likewise though, no doubt.
I think they'd have thought of this contingency.
As n0nce said, you need to re-request to be a validator, and there is some kind of timelock on your staked ETH as well.
I assume this is for the express purpose of allowing some time for a new fork to get started, and make it difficult for new validators - e.g. the malicious validators who mixed their previously staked coins to allow them to double-stake - to subvert the new fork.
So, the trick they are using to simulate the functions of PoW is basically a democratic consensus system. It's not a bad idea, but IMO still not as good as PoW, which is much simpler, and with no such attack vector. You cannot kill a fork in PoW, only not contribute to it.