[cafard]

A university police force has police powers within a university campus--it can arrest people. I will add that in my college days, the university police did not carry firearms, though now many do.

I imagine that the custom arose because Anglo-American universities tended to be rural or at least in small towns, unlike the continental ones. Remember that universities until about 1900 were exclusively male, so that one was gathering a large number of young men at prime hell-raising age. A college of 500 students might generate more work for police than a town of 2000 citizens might otherwise have, or care to pay taxes to police.