Who counts your posts for when renting your signature space? How do they trust that you had their signature all the time? How do they know that you haven't spammed in any of the posts?
I imagine that some of it is an element of trust, but there will be spot-checks too.
Ultimately spam is beneficial for those who run the sig deals, however spam is damaging for advertisements in sigs in general, so that's why those who run the deals take on the responsibility to direct its users.