A developer chooses to develop something and give it away for free. Why they do that is up to them... perhaps it's ideology? A sense of charity? Community?
Whatever it is, though, they're choosing to release that work to the world. And barring a non-commercial use clause, they're opening up that work for use in for-profit settings.
That's a choice. By definition it cannot be exploitative as there's no coercion involved.
If the developer doesn't like it, relicense the code to bar commercial use (gSOAP is the first example that comes to mind that does something like this), use the GPL to discourage commercial use, or don't release it. These are your choices.
https://www.google.com/search?q=define+exploitation
Exploitation: the action or fact of treating someone unfairly in order to benefit from their work.
It comes down to whether you think it's fair or not, which is a question of morality.
The work is being released as open source. The developer has already chosen to give up the rights to any profit they might obtain from commercial use of the software. If that isn't desired there's any number of alternative licensing schemes that would require compensation for commercial use.
They aren't being forced to work on the software. They can contribute as much or as little to it as they like.
So what would make it fair, exactly?
I didn't ask this guy to create the software and I am not asking him to maintain it either. If he abandons it, I will have to reevaluate by business but that is the risk I am willing to take.
So how and where did I exploit anyone? Had I chosen to hire a developer to re-write the same functionality provided by that software and pushed the cost to customers, would that be better? For whom? How? Would it be any different if we were a huge company? Why?
I just don't want to keep reading this stories of open source developers burning out.