Sorry, I don't agree with this at all, and I think that is part of what the "Soylent Green" comment was meant to address.

People aren't giving "free labor", the sites are tricking people into working for free with no claim to earnings. I think this is a very important distinction.

You are correct that the sites would be worthless without the users time investment, but you should also be aware that the users have NO claim to profits, earnings, compensation, etc., other than in the "privilege" of continuing to donate their time for free.