I don't think it's exploitation.I also used those sites for work and some off my client's paid me enough to at least pay the bills.Jobs are scarce and if we keep on complaining about not getting paid enough,sites like these would cease to exist and then no one would have an opportunity to make some money for thier game art.
It's up to the artist to decide whether he/she should take that job that pays poorly.I think that some people which are exploiting artist's have no idea how much work goes into creating game art.It's not a 9-5 job.Sometimes we have to work around the clock to get things done.