Many people's moral systems (and our legal system) are typically deontological. That is to say, the consequence of an action (or scale) aren't relevant to whether it's okay or not, so long as the action doesn't break any particular moral rule.

There's no rule against learning from books in general. (Maybe only for some particular books.) There's no rule against using tools to read books (it's okay to wear glasses)

The only deontological argument I can see against training LLMs on copyrighted data is that some people think it's morally and legally wrong to make derivative works (such as fanfiction) without permission, and the weights of the LLM could be seen as a derivative work.

This may sound stupid but it's the same argument people use in favour of adblockers. When ads were first introduced to the internet, it was understood that different people could view the internet however they liked and they could choose a "user agent" to display content to their preference. So ads were just a nuisance but could be worked around easily -- it was the ad provider who was the fool. Now I often hear people say that ad blockers are unethical; they deprive content creators of their income, or they're deceptive, or criminal. This may indeed be true. But at no point in time did any moral rule suddenly change.