ISPs may block sites to prevent unauthorized copying. It’s not a punishment for past wrong-doing. I’m not sure about the details, I think this differs a lot between jurisdictions. But basically, as ISPs they are involved in the unauthorized act of copying. Their servers copy the data to the end user/customer. So, they may be on the hook for infringement themselves if they don’t act.
Again, I am not aware of Meta sharing the copyrighted books in question. So, I don’t know what the legal basis for blocking Meta would be. If ISPs block a site without a legal basis, they are probably on the hook for breach of contract.
IDK on what basis the sharing of Meta’s LLMs could be stopped. If anyone could claim copyright it would be Meta itself and they allow sharing them. (I have doubts if AI models are copyrightable under current US law.)