The campaign manager has nothing to do with it. The campaign manager will look for the posts he writes and he will get paid only if he writes five posts in gambling boards. If he does not, he is unlikely to get paid.
But will you choose someone to promote your brand that calls it highly unethical? Do you think the owner will be happy that he is paying for someone that doesn’t like the brand? I believe the manager doesn’t know this fact when he accepted the user since he is really contributing on other aspect of the forum aside from gambling.
For some reason, I believe this thread should belong in the gambling board since the main question in the OP is why do you think the Gambling board is unethical. You can consider the title as clickbait and I used it because almost nobody has the same point of view as him. I already explained why it should depend on a person's point of view.
This topic will surely revolved around the user that you include in the title since the quoted post you provide is a heated argument regarding the user perspective on gambling against the ironic signature campaign that he is promoting.
Being OT here, Gambling is a legit business with licensed to operate. I don't see the point on how it's being unethical while it's probably fair to players. Gamblers have a fair chance to win and lose here. It's our choice on how do we spend our money for entertainment.