Can instructors fail honest students when tests are too hard because most students are cheating?
I agree that the policy seems to be that using previous years' tests to study is allowed. However, if this is the case, the faculty should say something and make sure that all students have equal access to previous tests. The fact that some students are unaware that using previous tests to study is permitted is a problem, as is the unequal distribution of tests. I could easily see a student who was marginalized from the group for some reason (sex, race, nationality, etc) not getting these unofficial emails, and then it would be an issue of discrimination. 

I would recommend the faculty post the previous year's tests to the website to make sure everyone has equal access and is aware that these are permitted study aids. 

As a student, I would push for a clear statement in class on whether or not these aids were permitted. If faculty refuse to make a statement or take any action I would recommend speaking to your academic advisor about the issue.