The name of the game
Casinos are there to make money. They do so from having punters walk into their establishments and lose money. (They certainly don't make it on the drinks or shows, which are there to draw in the punters and help them lose money.) In the case of pure chance games (i.e. roulette), this happens by default because the odds are set so that the house will win over time. With games where skill is involved, they rely on the house edge, and the fact that most of the punters aren't very skillful. Card counting, or high levels of skill, can enable a punter to beat the house and reduce their profits, something they want to prevent. They like having the *occasional* big win - it encourages the punters - but only occasionally and preferably in someone else's casino.
As private establishments, casinos can choose who they admit (provided they don't do it along racial lines). Basically, anyone who wins against the house on a consistent basis is going to end up on the banned list, regardless of how they are doing it. Card counting isn't cheating - it isn't breaking the game mechanic, just being able to work with it better - but as far as the casinos are concerned, the only distinction is whether or not they can get the law involved.
So, in Nevada they have a law that enables them to prosecute people who use mechanical/electronic aids to increase their chance of winning, regardless of whether that actually constitutes cheating. It's a testament to their lobbying power in that state. If they could codify a law criminalizing card counting in general, they would be lobbying for that too. If they could prevent *anyone* from beating the house, without putting off all the punters, they would do that as well.
To sum up, casinos are expensive fun, and they have no shortage of customers ready to lose money to them. If you are able to take money from them instead, they don't want or need you as a customer. Play with friends - it's cheaper.