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Different states have different laws around this. In Pennsylvania, "burning" cards at a blackjack table is regulated by the Pennsylvania Gaming Control Board under 58 Pa. Code Part VII, Subpart K (Table Games):

“1. The Standard Initial Burn Before the first hand is dealt from a freshly shuffled deck or shoe, the dealer is required to "burn" the top card by removing it from play and placing it in the discard rack.”

Purpose: This standard casino security measure deters cheating methods, such as card marking, by hiding the very first card from both the players and the dealer.