Comment by blindstitch

The burden of proof on the professor is so high there that the result would be nobody ever getting accused of cheating, which already happens because the graduate students that do most of the grading are not paid enough to do detective work on short papers. In my uni it works like this: the student is given a 0 for the assignment and if they wish they can go to Academic Affairs to contest it and get a formal review process. Most do not escalate it because this is a rare event for someone who is cheating their way through undergrad and can be seen as a cost of doing business.