Couldn't you replace the exchanges in question with major PoW miners?
I could, but my point is PoS give lots of power to centralized exchange. Not only they control user fund, they could use it to manipulate the network or earn more profit (by staking the coin) while user only receive none/small percentage of the profit.
The exchanges in question hold customer money, and customers could trivially move their crypto to their own wallet, or another exchange.
Customer could do that, but it's likely the attack will be successful before customer know the condition and there's possibility exchange deny the withdraw temporarily under various pretense (such as "network upgrade").