Computer infection by viruses either happens because of bad luck (visiting a page concealing malware), poor computer literacy (opening an attached file) or plain negligence (logging with administrator rights to perform mundane tasks). Similarly, identity theft can be encouraged by negligence, especially when one forgets to log out after having used a public or shared computer.
Having found himself sitting in fron of computers with the previous private account sessions still open, Jeff Rosenberg decided to“teach a lesson to negligent students” in a manner of a highly questionable nature and legality:
I can’t tell you how many times I’ve gone to the library or a computer lab and found someone still logged into their user account. It used to make me angry. It used to make me wonder what was wrong with people. I don’t know if they just forget or just don’t care, but either way they deserve to be messed with for their stupidity. It took almost a year of my life to find these and to some degree I feel bad, but I’m pretty sure these people won’t ever forget to log out again.
In the USA, the Identity Theft and Assumption Deterrence Act makes the possession of “any means of identification [...] used without lawful authority” a federal crime. There is probably very little ground for accusing Rosenberg of identity theft since he never came in possession of the students login informations. But there is certainly something against vandalism.
Anyway, take heed and read the entry explaining Why You Should Always Log Out.