Because the former is preposterous. It was a pre-fab "teachers" test from their textbook publisher. It sounds like the perfect thing to take the night before the real test to see what you may need to look over one more time.
The latter is less preposterous, but still in the wrong mind. Is it the student's job to disclose what they studied? Frankly as long as they didn't actively steal their professors test I don't see how they can be put at fault. They studied hard, studying extra material, and got lucky when their professor decided to forgo doing his job and mailed-in the creation of his test. So now it's their fault for not telling the professor "hey it seems you copied someone else's work"?
These sound like regular college students in a 600 person business class just trying to graduate. They're not the morally bankrupt scourge of the earth, and your damning evidence against their employability (or apparent lack thereof) is based on them not coming forth because of a study guide?