It's a weird sort of exploitation, because OSS maintainers who don't think they're being adequately compensated are free not to participate by volunteering their time.
- Is well-compensated while working for a for-profit company where the codebase is proprietary.
- Works in open source but is poorly paid or not paid at all.
[1] That they might prefer to work on open source is their problem, and not one for the economy, I think; I say this as an open source developer who gets paid to work on open source and prefers this over working on proprietary software.
I think it's a problem for the economy, not because of individuals' preferences, but because I think there is an overall economic benefit to work being open source.