By betting when they are banned from betting, and they know they are banned from betting, footballers are already showing they aren't the smartest bunch. The reason for blanket bans it that it's just a lot easier to police than reviewing every players account for rule breaches. And in age where you can lay odds for first throw-in, first free kick, first booking etc. it's better just to keep things simple.
In many businesses there are rules not just to prevent impropriety, but also the appearance of impropriety. If they really feel the need to degen their wages away every week there's a bazillion things to bet on that are allowed.