Sorry, but yes; self-plagiarism can be a copyright problem. Publishers usually have you sign away your copyright to them before they will publish your work. They can then legally prevent you from publishing your work with another publisher or yourself. Sure, the author usually retains a moral right to be identified as the author, but the right to profit from the work can be sold to others. If the author creates another work too similar to the first, i.e., they plagiarise themselves, then that can be just as much a problem as if anybody else plagiarised them.
The only entity that can't plagiarise a work (from a legal, copyright perspective), is the copyright owner, and that entity is often not the author.
Note that I don't believe this is a factor for Prince and Mooney since it seems that neither of them have sold their copyright.