Invisible changes are trivial for a pirate to undo (once they know what is going on). With visible changes, the pirate would have a significantly more difficult

Invisible changes are trivial for a pirate to undo (once they know what is going on). With visible changes, the pirate would have a significantly more difficult time to erase the watermark. My guess would be that, with only a single copy, he would have to destroy the contents of the books a huge amount to hide himself, where as the publisher would only need to destroy a very little amount.