I think a print server at a university is pretty obviously not fair game. If you forget to lock up your business overnight and someone walks in and prints off a couple thousand pages using your printer it's definitely illegal.
Your "forget to lock up your business overnight" scenario isn't a useful analogy since entering that business would be at least trespass and possibly breaking and entering (no physical breaking needs to occur), so anything after that obviously is a result of exceeding access.
Riffing off of my older analogy, if I entered a library and asked the librarian "Can you please print flyer.pdf?" and the librarian did so, is that "obviously not fair game"?
To be clear, I'm considering only the act of printing rather than the contents of the flyer; it could be a single pixel or the word "LOVE". I suspect that this is ultimately some sort of mischief since it's unlikely he had any plans to travel to each location to pick up the output, but I'm having a hard time seeing how this is hacking.
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.
I agree with you on the ethics/morality of it all.
That literally never happens. It's possible in theory, but in practice, if you encounter such a thing and send it a print job, you know you're accessing it without permission.
If you don't have permission and you know you don't have permission and you still do it, that's illegal. Analogies about librarians don't help.