At the biggest universities, police pre-date that law.

The reason is obvious when you consider how large many universities have become. If you throw 50000 20 year olds into a 3 square mile area, there's likely to be a lot of crime that happens. Sexual assaults, narcotics, and thefts mostly. There are, of course, more serious crimes that happen as well. In all that chaos, these universities have an obligation to keep order.