Using merit restrictions is an easy way for a lazy manager to appear to be doing his job.
Actually this topic was brought up a few weeks back and I specifically remember Hilariousetc stating that having a minimum requirement for joining will only encourage people to buy/trade merits.
Merit shouldn't be the only requirement: the campaign manager still has to check the post history. Look at is as a pre-selection: if 10
earned Merit points are required, more than 99% of the users can be rejected instantly.
Without Merit-requirement, I wouldn't allow Members to join a campaign anymore. There are too many of them, and their post quality is too low to be worth my time checking dozens of accounts to allow one to join.
With Merit-requirements, I can reject the majority, and for the rest I'd have to check if the Merit was earned with real posts (and then still go over the post history of course).
Theymos doesn't even need to require it. All it takes is for the managers to want to implement such a thing, and I don't see why they shouldn't.
I can think of a few reasons why they don't want to:
-they don't care
-it's less work and more money if they accept everybody
-there is barely any punishment if they pay spammers to spam
-their own English can be insufficient