It did happen. I was there (Laurentian University in Sudbury Ontario), teaching in the Department of Physics and Astronomy and my former colleague, N.I. Robb now retired - and an extremely well liked prof btw-, did just what I described.
In this example he plagarist has no dicincentive to plagarise again from some other source.
The nice thing about the zero to the stronger student without consequence to the weak student is that they didn't go down together, which tends to make the affected more likely to go confront the issue.
Sure, what I'm saying is not based on any hard data except my years at uni and the fact that a physics professor in my campus did the exact same thing.