I'm afraid that those bounty programs may create an expectation that all contributions need to be paid. If someone makes unpaid high-quality contributions for a long time and then sees someone else being paid for a minor fix, it will leave a sour taste.
On one of my own projects someone randomly added a bounty to an issue that was mostly an upstream issue. Of course the original poster had good intentions, but didn't fully understand this.
I can't imagine it working too well anyway. Valuable work isn't done by drive by contributions very often. It usually takes someone a very long time to become familiar with the project. The first few contributions are likely wasting the maintainers time but over time they become valuable contributors.
That's the hope is to bring some more people into OS contributions who haven't tried it out before.
I have funded a couple issues at like $25 that were marked "Good First Issue". And I've had people reach out saying they were excited to have done their first pull request.
On one of my own projects someone randomly added a bounty to an issue that was mostly an upstream issue. Of course the original poster had good intentions, but didn't fully understand this.