Counting cards is not cheating. It is using available information to influence betting decisions (i.e bet more when dealing with a favorable count). Casinos are allowed to ban people for assuming you are counting cards (was banned from the Barbary Coast Casino - should have known better, they were notorious for banning counters). The reason they can't punish you legally is that it doesn't meet the criteria for cheating at a casino game (try that and see what happens). So it's more of being able to decide who you will provide service to as a business.
Edit: I wasn't banned from the casino. I was told I could play any game I'd like to as long as that didn't include blackjack...
Having said that, the glory days of card counting are long behind us with 8 deck shoes, continuous shuffling machines, restrictive bet limits (reducing bet spreading), active casino countermeasures to detect counters, and no mid shoe entry at some places.