most of them speculate if doing more campaign their possibilities to earn money bigger than they selective accepting campaign. as we know in few months ago there are alot scam project , even that looks very good and team developers qualified. but till reward paid , we could not say it is good campaign or not.
When it comes to rewards, then everyone has a different assessment of this, because each project has a different allocation and splits the allocation in multiple campaigns so the revenue will obviously be different, so it's not a scam in my opinion.