They are offering bonus for reply on the thread which is not related with forum.
As I could see they are not paying for posting on their ann thread. They are paying for surf. On the thread they are taking proof who are the owner of the surf.
You can do giveaways off-site and link to the giveaway page in a thread, but you can't give people any bonus for replying to your thread.
I think it will be clear if you can see it once again. If yet it is really paying for bumping ann thread once system or moderator or admin would be able to see will lock the thread and ban all the participants.