Merit sources are chosen by admins on request and nomination from local board members, if there is something wrong with how they use their power then such cases should be investigated and MS exchanged if proven that indeed rewarded posts they shouldn't.
This is how merit laundering happens: I post a normal or high quality post, then someone gives me 50 merits, so I'm left with 25 smerits, then I send 25 smerits to another compatriot, then he will receive 30 merits from merit source, then he sends me 10 merits and so on. I would say that most of the times, local board members send inadequate volume of merits to their compatriots. Btw it's hard to argue whether person deserves 1 merit, 2 merit or 50 merits because everyone can back it up simply: I felt so.
However most of signature campaigns need to posts in global boards, so if they're shitposters, they should be not in campaign as the managers will reject shitposters.
Managers don't reject shitposters. Sometimes signature campaigns have low pay rate but need to hire a lot of users, then they are unable to hire high quality posters and out of 50 slots, only 30 is filled. What managers do in this case? They simply hire low quality posters because there is no high quality poster left who will agree their payrate but there is a gap to fill.