15 quality posts is most definitely obtainable and a lot of users are already doing this and more. In fact I'm quite surprised that they don't require more posts.
At what point does it become unreasonable? Requiring 30 per week? 50? More? I do think these posting requirements fundamentally change the idea of "getting paid to post stuff you would have anyway."
These posting quotas remind me of the ticket quotas that American police have. At the end of the month, it's very easy to get a traffic ticket because the police are
required to issue a certain number of tickets regardless of actual infractions.
The parallel here is that posters spam at the end of the week because they are required to...
1. Only allow trusted and pre-approved campaign managers - who won't tolerate spam - to manage bounties/campaigns. There are way too many managers who don't give a shit about what their users are posting and will pay them anyway.
2. Only allow pre-approved campaigns.
Whitelisting is a huge time suck for everyone involved and introduces all sorts of centralized trust. I think Default Trust is a horrible system already, riddled with conflicts of interest. Establishing "trusted" managers and "trusted" campaigns would just expand on that.
Free markets and forum guidelines/moderation that actually address spam: yes.
Centralized trust and gatekeeping in the marketplace: no.