Won't this just encourage maintainers to sit on issues... just waiting for the bounty to increase? They might even purposefully introduce painful but simple bugs just to push consumers to pay to have them fixed.
Open source operates in a marketplace today (just not an economic one) so market dynamics would flush this out imo. Intentional bugs, stale development etc would lead to user churn and/or forks. I'd argue faster within OSS where users can see how things are handled.
Having said that, I certainly think the risk of it and fraud more in general increases once an economic marketplace is formed. However, I think those problems are then "good" problems to have and easy ones to fix, e.g flagging repositories with unusually high ratio of repeat pledges, velocity of bugs etc.
An important distinction between Polar vs. traditional bounty services though and regarding your point is:
Everything goes through the maintainer(s) and we're equipping them with the insights, capital and tools to reward their contributors. While bounty services allow anyone to post a bounty and anyone to pursue it. However, ultimately it needs to be reviewed, merged and maintained indefinitely by the maintainer(s). By ensuring it all flows via maintainer(s) it:
1. Rewards maintainers as well – key given their efforts
2.Guarantees maintainers are aligned & behind pledges first. Combined with in control on how to expose, delegate and reward it. Avoiding unnecessary friction, bad faith and overhead if such alignment is first made during PR.
If a project became notorious for pulling stuff like that, and rejecting pull requests, people would most likely just make a fork.
I guess that also raises the issue of who gets paid if a third party pull request fixes an issue.