They do this because some players attempt to con the casinos by making wagers on the opposite side of the board at these kinds of games in order to fulfill the wagering requirements. At the roulette table, for instance, players can wager 100 credits on Red and 100 on Black. If Black wins, they will receive 200 credits and lose 100 credits, and vice versa. They place bets, but even if they lose, they do not forfeit credits. The "pass line" and "do not pass line" bets in craps work the same way. The most natural answer to this seems to be for other casinos to audit the player's past and deny him a payout if he met the wagering criteria by playing on opposites.