You have it wrong though: they get plenty of compensation through reputation, clout, and a feeling of power over their peers and being heard and helping out in a community that they love (or some combo of these). That's why they do it. Why is that not a fair trade without money exchanging hands?
Growing up, I used to mow an aging neighbor's lawn because he was basically couch-ridden. It made me feel good to help someone else out. Was I being exploited and should I have demanded payment, or was that just a fair trade that I volunteered for and could quit at any time?
Or look no further than HN. Participating on this forum writing content and enriching YC's coffers is "labor". And our compensation is procrastination and venting and feeling some satisfaction for scrawling our opinions online. If we didn't like that deal, we wouldn't be here.