[irinotecan]

Because in reality he could have gotten to 99% but never to 100%. There was always the chance that a student genuinely aced the exam on his own accord but would get accused of cheating and expelled anyway. That would be disaster for the prof and the school if the kid turned around and sued them. So instead he opted for a typical prisoner's dilemma to weed out the cheaters. And notice the carrot for that, take an ethics course and all is forgiven. Not, you will fail this class but be thankful you're not expelled.

[iwr]

The cheaters(?) would be visible if they do significantly worse in the second test, compared to the first.

[sukuriant]

That's not entirely accurate. Any number of things can affect a grade on a test. Included in this list is: personal issues, slightly (but sufficiently) different content, ambiguous questions (remember, these new questions were made in 90-something hours).

[cantbecool]

Bimodal distribution exist when an external force has been applied to a data set.

Now for a what if. What if the students apply themselves and study, but the class ends up getting around the same distribution of scores?(Where the median is a 90.) The professors theory of cheating is thrown out the window.