Maybe some campaign managers feel pressured in to filling slots therefore allow many things to take place which they normally would not. It depends on the campaign manager and also on the business being promoted. I agree, lowering payment and bonuses will probably bring an even lower quality of post from spammers but there is no easy solution except to self-moderate threads in boards where signature spammers and scammers are running riot.
It's not up to campaign manager all the time I think. Sometimes, project representative also mention some rules and among them, I have always seen they have a high number of post requirements in gambling sector. Personally, I don’t like to force users to post on any section but projects requires a minimum number. Campaign manager can recommend to lower it which I did too but sometimes, you can't follow your own. You have to hear the project owner.