Some casinos set a max payout instead of a max bet. If the max payout is 1 BTC, you can still put 1 BTC on red, but you can only put (around) 0.027 BTC on zero. It becomes slightly more complicated when someone bets on multiple fields at once (meaning up to 37 separate calculations of max payout).

My main point: it's better to have a maximum, so users can be sure you can afford to pay whatever they can win.