Most times I don't have to blame the casinos because most gamblers who go for this bonuses don't even have the patience to read the terms and conditions before going for it because if they do, then they would have understood that it wasn't going to be an easy task.
Nevertheless, I would prefer that the casinos make this terms very fair and not extremely against the player.
We can't blame the casino but must check what we've done to know what happened. If a gambler wants to get a bonus from an affiliate, they really have to be able to convince people that the casino is good and can be a place to gamble. However, people think that sharing affiliate links can bring them a lot of bonuses but that's not the case. They can get a lot of bonuses if people who join under them use big money to gamble so the bonus will also be big. But if they don't, they won't get a big bonus or even get anything.