Should students have access to past exams or an exam bank?
As a professor, one has to assume that all past exam questions are available to students. Some student organizations debrief members after exams and capture the questions for such an exam bank. 

Some professors realize this and take action accordingly, not reusing questions, or not using enough of them to matter substantially. Others think, probably mistakenly, that they can set rules forbidding the use of such "resources". The rules will be broken by at least some of the students. I don't know of any effective way to actually prevent access to old questions. So, just assume that they are available. 

My own opinion is that the professor should just assume that it will happen and, if they want to make the test taking "fair" will make old exams available for study. Some even go so far as to give out the final exam on the first day of class, to be taken later, though I suspect that not many do that. 

But old exams are useful for honest students to study from if they don't expect to get those questions on their own exams. 

Of course, coming up with new questions each term/year adds a bit of work, but if the course is so "stagnant" that an exam from a few years ago is still being reused then the prof needs to do a better job.