It might be interesting for them to keep track of how long it takes the black market to reappear, and see whether the negatives that come along with allowing criminals to monopolize the business of dealing drugs to people in the area reappear also.
drug tourism was posing difficulties for the city
The students who were banned were those visiting from out of country so it's possible to argue that the students performance is increased by preventing them from being drug-tourists. Not sensible, but possible.