I think they offered you the money, thinking that you are a high roller and a good target for making a good profit. I also assume that you used a similar tactic with your Stake account and that you reached Diamond as a result of that.... and not because you played Slots the whole day. 

In any way, well done .... most people would say that they deserved it.. for trying to pinch highrollers from other casinos. I do not blame them, because the competition for the gambling whales are tough.