I know for a fact that Harvard gives B’s and C’s 
That would make an A average impossible.
One of my kids teaches at a public university and has run into the same thing. But if it happens at Harvard, it fits a preexisting narrative that the public doesn’t fit.
Curious about the title of this thread: was the student involved wealthy and did the student offer money? How is “wealthy” relevant? And was the student “arrogant”?
Sometimes grad TA’s are inexperienced and don’t achieve credibility with students. Often this is not the case, but when it is, the grade might be questioned- at any school.