I'm not an expert, but I think they'd probably have to negotiate additional rights with the authors, even for print-on-demand.

everybody wins. amazon, publishers, authors, they can all get more revenue. Especially for things like novellas, short stories, mangas,etc.. where mass market publication doesn't make sense. or to gauge interest prior to mass-market publication. But for existing works, you're right, that might be a pain, especially then the authors are deceased.