As a professor and university administrator at a nationally renowned university, I can say that efforts are being made to deal with these types of situations, at least on our campus. Albeit, if the student is not in the program of the professor and does not takes any of the classes the professor instructs, then it does not technically become an ethic issue. The topic is really a sticky situation, especially on larger campuses.
As it being said as one of the "perks"... I, too, have heard many professors say the same thing, especially tenured professors.
I do not agree with it, but it constitutes a totally different element.