If they would freeze the stake of any other account, it would be the sure death of Steem and I'm pretty sure that they are aware of this fact. None of them would take that risk. I'm also not sure about legal consequences in such a case. I guess the community (there are more witnesses) would then fork the top ones out.

It was a temporary action that is only related to Steemit's stake. Any other billionaire is free to join Steem and vote the witnesses out of the top. There is no way to stop that in a decentralized network.