All they need to do is check if you're cashing out more chips than you came in with.
A buddy from out of town, or a losing regular, or a poker player who the casino doesn't care if they win. In Vegas some casinos' chips are negotiable, officially or unofficially, in other casinos.
They rightfully passed legislation banning such practices. The idea is that if casinos and bookies refuse to cap your downside, they have zero right to cap your upside.