If someone is in a pay per post campaign then he will try to post the maximum number of posts counted in that payment period,hence creating spam.
just the same when some fixed signature campaign have a rules which only users with 40+ constructive post will be eligible to receive a payment,the people who join this campaign will going to reach the eligible point, there's no different at all
and some signature campaign hire a manager to do a selection on participant's post so there's does exist people who handle this whole topic