I feel like you're missing something basic. The professor classifies this as a "cheating site", and his entire article assumes this classification, but the site portrays itself as a study aid. If we skip the editorializing, what the professor actually did was seed misinformation on the Internet to see who would fall for it.

Would you feel differently if he had edited the Wikipedia page for his subject to see which students used it as a study aid?

The equation of "you found this info on the Internet instead of in the approved course material and therefore you're cheating" doesn't seem completely solid to me.