Both Meta and ChatGPT used books3, it's functionally the same type of case.

Why should ISPs deny their paying customers access to Meta sites or sites hosting LLMs released by Meta? These customers have contracts with their service providers. On what grounds, would ISPs be in the right to stop providing these internet services?

In the countries where ISP blocking happens, its usually because a copyright holder has sued and demanded blocking at the ISP level and has won in court. Then, the government begins the path of working with ISPs to block the site.

Unless you think most governments that do this do it arbitrarily? No, they do it because a copyright holder sued, like the New York Times has. The NYT has not demanded ISP-level blocking, but that does not mean that they couldn't. I can't speak to their choice not to do so other than it seems that companies only save that for truly altruistic groups, and rarely do it for other big corporations.