Whether card counting and other tricks should be allowed or should be made straight down  illegal to use in the casino table games industry has been and will be argued about for many years to come. It, however, does not stop players from implementing this casino card counting tactics during their rounds of games. From the one hand, players get to succeed in their games by using nothing but their own brain, so it seems to be irrational and just wrong to take this opportunity away from them. From the other hand, casinos lose money, and a lot of them, when players start winning frequently so it would seem natural for them to want to issue some kind of law or at least legal restrictions in order to forbid such behavior.

At this point, counting cards is not illegal, however if your are noticed to be doing that (how they notice what and how you think is totally different story) casinos security guards will politely but quite encouragingly suggest you leaving the premises and of course will assist you with that.

Online card games, of course, cannot have such a restriction (there are plenty of other ones), so online casino card counting is quite a normal playing tactics. There is even a casino card counting software available and it is strongly suggested you download and use it at least for educational purposes.