Why do professors like giving out bad grades.

Was kind of reminded of this article: http://articles.latimes.com/2001/jun/10/opinion/op-8654
Since when did A- or even a B become a “bad” grade? Oh, right. When suddenly half the class is getting A’s. But then how do you differentiate between students if the average grade for the class is what is supposed to be the best grade for the most stellar students? At that point, there’s really not much point in even having grades.