Managers are mostly responsible for such posting. For example, even yahoo, sylon and other worthy managers wants from their bounty members 20+ posts per week. It's not easy to create 20 worthy posts.
If it's difficult, then don't apply. Is that not a simple principle to follow?
It's not easy to be a full-stack developer with minor programming experience. Campaign managers should take responsibility, yes. However, at the end of the day, if the campaign member is not sufficiently qualified then they should be booted out. Furthermore, they shouldn't have applied in the first place. If all that they can output is bad quality work, then that's not something that should be rewarded.
TL;DR: You should not be paid for a job you cannot do.
While it may be clear to you that if you cannot post 20 quality posts in a week you should not apply, this is not clear to others. From the point of view of a bounty manager (this is an assumption - I do not know with certainty as I am not one) the more participants and the more posts the better because the signature will be more widespread across the forum.
I acquiesce that because of the financial incentive, this [rank] should be limited. However, that's an issue that campaign managers should take care of: they should punish poor posting quality. And if they allow spam to manifest on the forum, then they themselves should be punished.
Again this is not within their incentive, do you really feel that bounty managers are all that much better than the signature spammers that they reward?
The easiest solution to kill another big chunk of the spam (which I've seen banded around a few times) would be either to make Jr. Member have a merit requirement or to make it so Jr. Members cannot have signatures at all. At least 50% of the accounts in signature campaigns are junior members. We cannot rely on managers to change their rules to only allow members plus because they've had a month and a half to do that and I think I've only seen 1 who has done that.
Disclaimer: I am talking about alt-coin campaigns, I have seen that many bitcoin campaigns are using merit to identify better quality posters.