I haven't visited the Boundless site, but I think it's not really as much about copy-cat books as it is about replacement reading material. If your teacher wants you to read about Hegel's theory of history in chapter eight of The Nuances of Philosophy, 13th Edition, for instance, their site has a non-copyrighted article that covers the subject, listed as a replacement for that chapter. So if you're only interested in understanding the course, you're set.

The problems and questions in the textbook are a different matter. That's one of the ways publishers force teachers and students to buy the new couple-hundred-dollar books instead of the cheaper older editions. They just switch the numbers of the questions at the end of the chapters around.

If a teacher cares about students getting ripped-off by publishers, they'll just create the assignments themselves, so no one has to buy the absurdly-priced new-edition textbooks. But, these teachers will usually assign non-textbook reading material in the first place.