It's likely that for a lot of writers, this is a supplemental form of income. It's a great part time job or a way to make extra money when you work for established companies with strong reputations and a large population of active consumers.
I doubt that many writers would want to involve themselves in the business side of the industry, particularly when enjoying this as supplemental income. I would question the wisdom behind entering into a business arrangement with strangers as well.
I think that some companies do more than others for their writers. The suggestion that all companies are "pimps" seems a bit off but I could see that applying to some of these companies that do little more than match up potential clients with potential writers. I don't think that they offer much to writers or clients.
I agree, but there is certainly value to having one person or a few people handle promotion and management so that writers can focus on not having to do that.