Seriously, telling them beforehand just puts a target on your back, and fair or not, it's harder to argue that any response they have is retaliation if you haven't actually done anything to retaliate against. It also gives them a chance to cover things up before any investigation that may occur, which protects them from punishment and prevents those harmed (i.e., you and your coworkers) from recovering any damages.
On the other hand, reporting them gives you some protection from retaliation, and chances are that they're violating the rights of coworkers as well, so they might not even know who reported them.
Of course there are exceptions to this, but they should be limited the rare cases where your employer/manager is actually a good person and they're new enough that they might not actually know better.