Amazon will filter certain kinds of book covers from the main search results. Usually these covers have implied nudity or are simply too overt for Amazon's liking. Therefore, people put tamer pictures on erotic books in order to avoid the filter.
There are hundreds of thousands of these books out there. The authors use cheap microstock because they write 2-3 of these stories a week. This is a hungry market, but they need cheap.
The agencies should create some kind of designation that retains a low price, but eliminates the confusion of whether an image can be used on an erotic book.
I'm sure authors don't want to end up like the ones Nicole Forni is suing.