What amazon could block is getting books from other sources onto a kindle. But there's plenty of devices. I use an iPad.
Oh, and to pirate textbooks. The issue is that an LLM-entered (as in in context) version of part of a textbook is something that I can talk to, write to, and have it judge my skills. Normally I'd have to find someone who'd be willing to spend a short time talking to me about a subject, and correct me, and who's willing to spend hours correcting assignments from me. Even when paying, that's essentially unavailable.
Now I take a few pages, let's say up to a chapter but usually less, load it into ChatGPT-5, tell it to ask me progressively harder questions when I activate voice mode. Or I take one of those for-teacher "how to grade X" notes, write an assignment, scan the whole thing into ChatGPT, and tell it to correct my assignment, justifying everything on the teacher note and deliver a final grade. I tell it to be way too strict, and this has helped me, among other things, get very good, and one perfect score on language certs. I can prove that I am fluent in 4 languages (en, fr, de, and my mother tongue). If we're talking anything but specialized language it's even true.