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.
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.
I would agree that competition is really tough across all gambling platforms, even in landbased casinos, whales are given special privilege because if he jumps in to join and move to their competition, it's done for them. I personally know a big whale here in my country that's why I can attest to the influence of this big whales and high rollers.
Not sure though as per OP how did one casino share or get his information though that's why he was offered the money as you described because in the other casinos that he mentioned perhaps he as a Diamond member already?