Yeah I responded to that the other day about a recipe app that tried something similar. It allowed people to scan the barcode of recipe books they owned to add the contained recipes into the app. They were busted.
I'm not familiar with Readwise but it appears they only store highlights? Publishers probably don't like that but don't care enough to do anything about it, whereas "uploading copies of books to the services to do AI stuff with them" is the subject of many ongoing lawsuits right now, so they'll be very interested if this takes off.