Counting cards is not illegal by law in most places, including the US and the UK, as it involves only mentally tracking cards without altering the game or using devices. However, casinos consider counting cards a breach of their rules and trust because it disrupts the casino's built-in mathematical advantage (house edge) and threatens their profitability. As private businesses, casinos have the right to ban or refuse service to players suspected of counting cards to maintain their business model and the appearance of fairness. If a suspected card counter refuses to leave, they could face charges like trespassing, but the act of counting cards itself is not a criminal offense.

Why Casinos Ban Card Counting

• It reduces the casino's profit by giving players an advantage.
• It can discourage casual or new players who expect fair and balanced games.
• Casinos want to maintain the reputation of games being mostly luck-based.
• It disrupts the balance of odds carefully calculated by the casino.

Legal Perspective

• Counting cards is a mental strategy and does not involve cheating through illegal manipulation or devices.
• Casinos are private property and have the right to enforce their own rules, including banning card counters.
• Players can be asked to leave or be banned, but cannot be arrested for just counting cards.
• Trespassing laws might apply if banned players return against casino policies.

In summary, counting cards is illegal in the sense that casinos forbid it and enforce bans, but it is not illegal under the law itself. Casinos protect their interests by banning card counters to prevent loss and maintain their business model.