There's a motivation to create posts on the forum because these projects have their activity requirements. If no one could add signatures or personal text to their profiles on bct many would loose their incentive to create new posts which could possibly force projects to look for forum alternatives with less restrictions.
But there's a delicate balance between having too many rules and potentially driving users away or not having enough, allowing spam posts to flood the forum reducing it's usefulness.
Merit still appears to be an effective way for managing and reducing spam and tweaks can always be made down the road to help improve its effectiveness but you also make a good argument for removing posts made in the bounties sections from the activity calculation. Perhaps making this change would also improve the quality of posts on the forum.