Counting cards is not illegal—in the eyes of the law—but almost every casino prohibits this behavior. Casinos frown against this tactic because it puts the house at a serious disadvantage.
Why It’s Illegal
You must understand the basic principles of counting cards to realize how unfair it is to practice this strategy. Essentially, these players keep track of the dealt cards to determine the likelihood of receiving a winning hand. The player could stand to win a very large sum because of the dramatic increase in their expected value (EV).
This tactic is pretty obvious when done solo. Often, a team of players will work in cahoots with each other—like seen in the movie ‘21’. Someone else will show up to make the big bets when they catch wind that the table is “hot”.
Casinos now choose to shuffle every hand, use many decks, or other anti-counting tactics. Their goal is to reduce the likelihood of this behavior. For the counter, it is very difficult to master the tactic in the first place. It is easy to get caught counting cards—if it happens your winnings are gone and the casino will blacklist you.