Betting on any football for anyone who works in football is banned, be it players, back room staff and potentially even those who just work for clubs. Essentially its something like the equivalent of insider trading for football because you can have information thats not available publicly. And if players were allowed to gamble it opens up the risk of spot fixing in the game where players deliberately get booked or concede fouls etc and manipulate the outcomes, that one yellow card may not influence that game itself but the accumulation can affect when their game is they're banned for etc.