Casinos in the United States are legal under the Federal Gaming Act of 1978. This act regulates all aspects of casino gambling, including the games that can be played and how much money can be wagered.
Bingo is not specifically mentioned in the act, but it is considered a game of chance and therefore is legal. Some states have specific lAWS regulating bingo, but in Pennsylvania, it is considered a game of skill and is therefore legal.