I'll refer you to the CAN-SPAM Act of 2003, which does not just govern unsolicited e-mail, but all commercial mail which the law defines as electronic communication (bulk faxes, etc.)
The printers are on public wire.
You had not done any crime by using Google to find them.
You obtained access to their open HP admin panel via public link with no password or credentials you had to pass.
You haven't stole any information and, furthermore, there is NO confidential information even to be stolen to start with.
On the top of that, you cannot even determine who they are (name, company, address, email, nothing?). They are totally undefinable sitting by a raw IP address. Sure you know someone is using HP printer. Can you get legally punished for that?
I don't think that taking advantage of a publicly accessible information is punishable by jail, especially since noone got hurt and no information were stolen, whether it is information someone made their living off of (Aaron case), or just totally worthless information as of what brand computer of printer is being used. It would be hard for a company to sue you -- (lack of merit)?
If Google got away with snooping private data from open Wifs (and I am sure they made some sort of use from all that gathered data, even if only internally), then I am pretty sure you wont get any heat for such a petite stretch of snooping people's printers.
another though: you may say that someone can sue you for printing a page using their material and toner, but thats too little of damage to even start with. However, arguendo, if you would get slammed with class action lawsuit, you are most likely a millionaire from your idea anyways :)
There's even less barrier to sending a junk fax, and that can get you fined and potentially jailed.
There have been case(s) I think (in USA) concerning websites where it was argued successfully that placing an non-password protected page available on the public internet was implied consent to access/use that service.
That seems the right way to do it. You can't then, for example, put up a website which enables printing and then claim that people who use it are financially liable for using that service.
That would be like putting a bench on a busy street and then popping up and charging people if they happened to sit on it - if they sit down, you can tell them they're not authorised to sit without payment, or you can advertise lack of authorisation (eg with a price list) but otherwise you're implying consent.
(also, how very considerate of you)
Now not every layman knows how to properly hook things up to the internet, but there is a definite implied consent in doing so. If the pages were restricted by password and we were bypassing it, or they were locked to an IP and we could spoof it, then there wouldn't be an implied consent to access the service being provided; but that's not the case here.
If you want to look at intent then it's notable that many listed are University addresses - people setting up those printers absolutely know what they're doing.
If you purposefully used excessive paper/ink or you kill the hardware with a broken firmware update then those things are definitely not authorised by the implied consent and would constitute vandalism.
I can see them getting you for spam, just as they can with unsolicited faxes I believe, but anything more than that? Seems a little silly.
To add to the printer/fax comparison, I have known people who used printers in different physical locations within an organization as a "fax machine" that was easier to use with a computer. Need to send some documents to the guys across the state? Print it to them.
The Federal Prosecutor threatening with 50years of jail time Or you can accept this no-brainer plea bargain for only 1year.
See also, 50 million posts on this subject last couple weeks since prominent software engineer suicided himself.