If I am a qualified and experienced editor of any sort, I personally do not want to work on some low wage. If I am qualified to become an editor of any sort, I'd be sending my job application to publishing houses where I get a steady and reliable wage along with other job benefits. I will not be applying to work for Peanut Romances that offer me promises of future payments (only if the book sells!) or free electronic copies of the book I worked on.
Perhaps the generally low level of incentives offered give rise to shoddy editing and proofreading? That's my guess anyway. I mean, if you offer peanuts for wages, what kind of job applicants will you get?