Unscrupulous publishers and editors. They are out there. But how do you recognize one? The advice sites always say that if someone asks for reading fees or other types of fees, to run screaming the other way. This, of course, is a red flag. But some preditors aren't that blatant. Some of them seem totally legit. They are interested in your work, they publish your work, but they don't pay you for your work. Now, not every place you get published will pay you. There are a lot of "for the love" places out there, and they aren't doing anything wrong. So how do you know if you've fallen into a trap? First of all, go with your gut feeling. If it doesn't feel right, move on. If they publish one of your books and then don't ever pay you royalties or discount you on author copies, they are probably not working in your best interest.
It's not always easy to recognize a preditor. And if you've been rejected a million times and are desperate, it's easy to fall prey. Trust me, I know.