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.
(also, how very considerate of you)
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.