Signature campaigns wouldn't want people signing up, not making any posts and then claiming their payment.
Minimum posts are essential (whether on a weekly or monthly basis). Otherwise, campaigns will pay per post and that creates bigger problems.
Firstly, one could simply reject payment given the quality of the poster or have an incentive programme.
What such problems would there be to switch to pay/post?
I'm willing to say that accounts enrolled in signature campaigns would be posting to the maximum most of the time anyway. Additionally, if you're going by a pay/post system, if the user in question decides to post less during the timeframe, then you pay them less in accordance. In fact this would be a favorable outcome considering the site would have the user spreading posts across multiple weeks for the same amount as one who would post for say, one week.
As an example, consider the following:
Alan and Bryan are enrolled in a campaign that is pay/post. It pays 1 mBTC per post and the maximum posts/week is 20.
Alan posts 10 in the first week and 10 in the second week.
Bryan posts 19 in the first week and 1 in the second week.
I'd be willing to say that Alan brings along more of an effect than Bryan.
Why do you think fixed campaigns are a problem, but pay per post ones are not?
Signature spam is a problem of users and managers, not one of the payment scheme used by the campaign.
Fixed campaigns force the users to post even if they're unwilling. This, in turn, could result in spam.