>I have never understood why a digital good isn't subject to the same protections as the physical one

Because digital and physical goods aren't the same thing. Equal rights principles presuppose that two things are equivalent. Unequal things can be treated unequally.

Each copy of a physical book is mutually exclusive, a library can't buy one copy and then lend it to a thousand people at the same time. Placing limits on how digital goods are distributed is actually how you restore some equality in regards to the property rights of the author.