The max bet you can put is often limited by the game or by the casino, remember, these are bonus funds and are limited by the conditions of the wagering. The max bet is also a way to ensure that players gamble responsibly. Players are more inclined to gamble responsibly with a free bonus when it is restricted by a limit, as opposed to spending all of the bonus at once and still owing money on the wager. The bonus funds are separate from the players account balance so this limits them from making a max bet that they cannot afford.