They do this because some players try to trick the casinos by placing money on opposite bets at these types of games to meet the wagering requirements. For example, players can bet 100 credits on Red and 100 on Black at the roulette table: if black wins, they will get 200 and lose 100 and vice versa. They place bets, but they do not lose credits if they lose the bet. It is the same with craps with the “pass line” and “do not pass line” bet. Other casinos will audit the player’s history, and if he met the wagering requirements by betting on opposites, they would not grant him a payout, which seems like the most logical solution to this.