Casinos are private properties, they can kick you out for any reason without having to justify themselves. Playing too well is one of these reasons.

Of course, kicking you out is the only thing they can do. They can't take your earnings, prosecute you, detain you, or use force unless you resist. Usually the player will be kindly asked to cash-in and leave.

Security can track your play patterns, your gains and losses. Card counting result in very characteristic patterns, and if your gains are too consistent to be explained by chance alone, they are going to watch closely. If you are just lucky, they will offer you free drinks, if you are an advantage player, you will be asked to leave, if you are a cheater, you are going to have problems.

Edit: I think there are some places where casinos can't just kick people out for no reasons. But anyways, there are ways of making card counting impractical. The use of continuous shuffling machines is one of them.