well... i am working at a broker and we meet also many "greedy" clients as you say. supply and demand. I am working in this business for 8 years now. in the past there were no bonus offerings by brokers and the spreads were higher. that bonus and more and lower spread and cheaper and what else do i get if i trade with you attitude started when scam brokers sprang up like mushrooms (having now reliable regulation and reputation, they had to attract the clients). bigger brokers had to follow and now we have this situation that clients ask for a bonus with the first word entering in the Chat.
but you are right Andrew. those greedy brokers are mostly the Market Making DD brokers. from one pocket to another the bonus will wander, since those brokers make the client lose. and if not, they make it impossible to being able to pay out the bonus or profit out of that.