They gave opportunity to expose about their brand name as much as they can all over the gambling board with negligible pay rate compared to any other signature campaign so its a kind of business at the end. 
At the end of the day, signature campaign is actually a way to do marketing of their casino and there is no harm if they want that their signature should be shown more than any other signatures in the forum. If they have a spam check mechanism in their campaign management, i don't think quantity should be a problem.
I've got mixed feelings about that campaign. On the one hand I think indeed incentivizes participants to make a lot of low quality posts, and I have seen some who write poor posts. But on the other hand, I have seen some pretty decent participants. The other day I checked that one I see quite a bit had written about 70 posts over the week and to me they are of decent quality.
Well, lets have an example if a person posts few posts a week and they are mostly spam posts and then there is another person who post 50+ posts in a week but all of the are good ones, then whom we will think is spam poster?
I think the quantity of posts have nothing to do with the quality of posts. Some persons may have a lot of time to write a lot of constructive posts throughout the week.