The issue is that many people in signature campaigns post only because they are in the signature campaign, and when the campaign ends and they are waiting for a new one to join, they are largely inactive. This is the wrong mindset to have - only posting for financial incentive - and it is very easy to identify the users who fall in to this category. Payments from a signature campaign should be seen as a nice byproduct of something you would be doing for free anyway - reading, learning, and contributing meaningfully to the forum.