But then, isn't the question one of harassment vs. blaming the victim from being offended?

If this meets a legal (and moral) definition of harassment (which your quote does), why should the professor be able to continue? 

You're asking me to comment on a situation where the professor deliberately is upsetting the student to prove a point. Even if the point was right (which I do not argue), why would that be okay? Why should the prof be able to continue? 

I don't see any difference between this situation and a prof who repeated used the N-word or the C-word. Note the use of the word "persistent." This indicates a situation where someone in authority uses their power and status to target a student. That is a very, very different context than exploring the issue in a neutral and scientific way. 

Targeting a student, for any reason, is never okay.