If you check very well, there could be more reason to it but the most important is payment, some casino don't like an external manager to handle the distribution of payments to signature participants probably because of trust reasons or others best known to them but it's also an opportunity for the participants to make use of the casino. I mean they are indirectly making promotional for the company, it's going to be a very good idea to have a good use of what they are showing others.

What I don't like about direct payment is the fee they charge from people, it's improper to pay your promoters money and ask them to pay for the fees. Some do white-list participants so they don't pay the fees but majorly they asked for withdrawal fees. There is one casino on that service board right now doing campaign and asking participants for $6 Bitcoin withdrawal fee, that's absurd thing to do; even if they are not going to allow free, $1 is enough to pay anyone there money.