They have the right to punish you if you acted any rebellious act to a product. Like, would they keep you just by proclaiming "Give Graal One Star if you hate the server lag!"
Have common sense. Why would they keep you if you would just let their server go down? Obviously, the result is negative so they will kick you out for sure 101%.
Criticizing in that way is NOT JUST. Its not even called criticizing, its called rebelling. You can just suggest not by bringing down a server. Really.. if you hate the server, JUST LEAVE IT ALONE. They aren't forcing you to play.