But copyright law does, because otherwise you have two copies.

This isn't theoretical, AI companies have finished lawsuits about this and this was the ruling.

The ruling was that what they did was within the law, not required by law in every detail. They cannot resell the copies. I saw nothing in that ruling that required their destruction, because it is not required.

A person can digitize their own books without destroying the original. So can Anthropic. They are choosing to destroy the books for easier scanning and trying to palm off the blame for it.