Authors can license textbooks instead of selling them, but do they?
I guess I wouldn't be surprised if kids these days (yes, I'm old) are agreeing to EULAs when they open their textbook apps. But I know for sure that tens of millions of people still alive today, purchased textbooks instead of licensing them. If those textbooks still exist, then the knowledge is attainable without any contracts, so there's no means of discriminating against computers.
Just avoid the weird textbooks (ones that require special software to read) and anyone's LLM can get around the problem you're describing.