What's the problem with letting the community work 'for free'? They are receiving benefits, even if non-monetary. Peer recognition, control, power...
If foobar.com's forums asks me to an un-paid moderator and I gladly comply, I have no right to decide 6 months down the road that I shouldn't have been doing that work for free and deserve payment for it.
Work laws are weird like that. There are certain things you can't consent to even if you really want to.
For example, you can't agree to work for less than minimum wage, you can't agree to work for free to get rid of a debt, you can't agree to have sex with someone in exchange for money.
These things apply even if you voluntarily sign a contract saying you consent.
If foobar.com's forums asks me to an un-paid moderator and I gladly comply, I have no right to decide 6 months down the road that I shouldn't have been doing that work for free and deserve payment for it.