Is it ethical to use past years' final exams that the professor hasn't explicitly given, to study for a final in the same class?
Perhaps this is a bit of a genuine ethical issue: should one use "game advantages" one might find, that one knows other "players" will not have? Of course, these questions completely ignore issues of the subject itself, learning, scholarship, etc. But, yes, there are advantages to "good test scores". That game is primarily (contrived) an adversarial one between "teacher" and "student", and any information obtained "legally" by the students is... "legal". It's a game! 

In that model, other students may suffer "on the curve". Not nice, true, but, ... 

I think the real answer is that, especially internet-wise, any policy that presumes secrecy of information is misguided. Sounds innocent, but, as this test case shows, there are "victims". 

My sincere conclusion, and basis for my own actions for 15+ years, is that everything should be made public. If someone can assimilate good solutions to all the exams for the last 20 years, ... then why shouldn't they pass the current exam? :)

(So, the kids who cram from the last few exams are merely being rational, assuming they didn't have to break-and-enter.)

The unasked question about whether the behavior of the "teacher" is inadvertently unfair is the interesting one. That is, lacking intent, can one be unfair? Yes, it turns out, by negligence...