Do casinos destroy cards?

The cards are printed expressly for the casino. Cards, like casino chips, are trademarked property. The last thing the casino needs is to expose itself to litigation owing to counterfeited property. Used decks are either punched (and sold as souvenirs) or are marked then shredded/cut and then destroyed.

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Is it illegal to count cards?

Card counting is NOT illegal under federal, state and local laws in the United States as long as players don't use any external card-counting device or people who assist them in counting cards. In their effort to identify card counters, casinos can ban players believed to be counters - sort of. Why do casinos not reuse cards? When the decks are removed from a game, casinos go to great measures to ensure they don't appear again. The cards are "canceled" to prevent single or multiple cards from being reintroduced into a game later on. It is simply a security issue to discourage cheating at the casino.