With regard to one of the more prominent essay-writing companies, Oxbridge Essays, I can inform a couple of your concerns, since I used to write for them!
When I commenced working for this company, I was assured that I would be writing model essays as "examples only" and that the company was strictly anti-plagiarism. However, a dozen or so essays down the line, they emailed me to report a "problem" with the bibliography in my latest essay, a 19,000 word Masters dissertation.
Apparently, the bibliography was not specific enough, since I had included a list of books/ publishers, etc, but not the specific page numbers of these books. I wrote back, inquiring why a so-called "model essay" would need to have such a specific bibliography, unless the recipient was attempting to plagiarise the entire work and pass it off as his own. I received no direct response to this question. Instead, the reply again requested me to make the aforementioned changes. I wrote back, explaining that I no longer had access to the resources necessary to do this, since I had to borrow many of the books from my uncle (a lecturer), who, in turn, returned them to his university library. As a graduate, you must remember, I cannot easily gain access to many copyright libraries! I also mentoned that they had emailed me more than 10 days after the essay had been submitted for changes, which was clearly past their deadline for requesting changes. Had the person I was writing the essay for originally read it, accepted it and handed it in as his own work, only to be told later by his tutor that the bibliography needed more precision?
Oxbridge Essays answered with yet another non-sequitor, stating that they had to check my essay for plagiarism before they paid me (even though they should have done this the minute it was initially submitted and despite the fact that my essay-writing style is transparently distinctive). I wrote back, explaining this, adding that if I was not paid by the end of the next month I would take them to court. I am currently waging a legal battle versus Oxbridge Essays to reclaim my money.
During this time, I have researched the company in greater depth and thence heard many similar stories. I subsequently regret having been used in such a way as a kind of "academic prostitute" and can only hope that others are not similarly duped. If I had been smart enough to research them carefully in the first place, I might have avoided these consequences.