Only because they have a return address. The idea that reading a book for free in one context is "legal" and reading the same book for free in another context is "illegal" is simply legal fiction invented by congress to prop up an industry. It's not a law of nature and it's not even logically consistent.

You are free to ignore bad laws. Read books for free. It's fine. It's what libraries are for, and it's what the internet is for.

The idea of "copies" is irrelevant. If 100 people in parallel all have a file of book on their tablet simultaneously and read it for free or 100 people all serially check out the same single physical instance from a lending library and read it for free the effect (and amount paid to the copyright holder) are precisely the same.