It's not "cheating", per se, but it is unfair.

How does an unaffiliated student get access to those prior year exams? Fortunately, the internet is levelling this somewhat.

When teaching at a college level, I used to go out of my way to provide any historical resources. I want my class to be fair to everyone involved.

I also used to redraft my exams and assignments every single year. This is quite a lot of work and a lot of professors simply won't do it. However, if you don't, then some students will take advantage of it, and your class isn't being fair to everyone.

If the instructor says you can't do it then it is cheating.

It's common for professors to not allow students to keep exams for this reason. Students in such classes do a "brain dump" by writing down all the questions they remember immediately after leaving the test to get around this.

Since you bring it up, I think studying questions that a company has been known to ask in the past is somewhat in poor taste, especially if the candidate gives the impression that this is the first time they've seen the question. Usually, when I've done interviews, I'll try to throw a wrench into the question and see how the candidate reacts if I suspect they've seen the question before.

Though in both cases, I think it's on the question asker (either teacher or interviewer) to make sure that questions change up frequently enough and aren't easy to study without having genuine understanding.