Good points.
However, they're not only checking by themself but with the help of their provider. In my speculation, the provider can be stored how the user activity until now everytime there has a problem for some like this most of the time always from sports bet players.
Meanwhile, my self playing livegames withdraw more than a thousand dollar don't have a problem comparing to some sportbet while get caught and their fund got lock who are deposit 150 or 200$.
Maybe the system is the same as what this forum does, for some reason, the forum imposes restrictions on users with devil IP's. Or from the connectivity of blacklisted bank accounts or wallet addresses, all of these can be the reason for how the system blocks accounts. IMO.
But, the case is over, we have seen how BK8 returns the deposit made by the user. We don't need to know how this system detects fraud, because if it were opened in a forum like this, it would become a weakness for the provider.