I think a better analogy would be if a library accidentally placed a paper copy of their members' records in the library alongside the books, you found it, and

I think a better analogy would be if a library accidentally placed a paper copy of their members' records in the library alongside the books, you found it, and instead of informing the library of the horrible mistake you took it over to the library photocopy machine and made a copy for yourself and showed it to everyone to humiliate the library for their mistake. Or if the printer was in a locked room in the back of a library, to be opened only with a keycard, and said "employees only" and you found out that the keycard didn't work quite right so you could walk right in and use it. It doesn't matter how easy the hack is. If you use someone else's machine in a way that hasn't been made publicly available AND it is public common knowledge that what you are doing is against the wishes of the people who own the hardware then it's illegal and immoral. If a bank exposes monetary transfer functionality by way of a public http GET request it's very stupid but it's still very illegal to use it to steal money from people.