You can think of it as free work, of you can think of it as people building something they care about. When I post on here I don't think of it as free work but more as contributing to a community I want to be part of.
Moderating is a part of community building and growing, however.
You're right. It's far more powerful than posting. Maybe they should be paying to be moderators, since they get extra influence over the conversations.
Voluntarily signing up for becoming a moderator for free borders on sociopathy. Some of these people are on a real power trip.