Not all open source developers are poorly paid. I think it's a free market, and I don't see that as a problem. I don't think open source developers are really being exploited, because typically they are capable of earning what they need if they want to. An open source developer can simply work for a for-profit company on proprietary software if they don't think they get paid enough, for example[1]. And the industry will pay what they need to get the software they need.

[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.