From a practical standpoint, while it's disappointing, I don't understand why they are so surprised that solutions were posted online.

I mean, if there are 5000 students in the course, for even odds of it happening, 0.5 = (1 - n) ^ 5000 -> n = 0.0139% chance per student, which seems like an awfully saintly ratio to assume.

Won't they be making the problem worse by drawing borderline cheaters' attention to it (http://en.wikipedia.org/wiki/Streisand_effect)?