Without any context, it's not possible to give a specific answer. It might be because some campuses have their own police force (not just a security department) and the city police do not have jurisdiction there unless they are asked to help, just as they would be asked to help in another city.
If there is a distribute at my private property, and an officer is aware (drives by and sees it) He does not wait for me or someone else to call or invite him on to my property.
If a student calls the police about it and says they feel threatened, do the police come or say they cannot until they are invited by the school administration? I would not want to attend a place with that type of agreement with police.
I would imagine that if a police car drove by your property and saw some violence being committed, he would attempt to do something about it.
There has to be a reason why police cannot just come on private property. It's possible that laws vary from state to state or city to city. I know that in our development, the police wouldn't come to remove kids who were smoking pot on our private property unless and until we posted that it was private. However, when I had some kids harassing me because they wanted to get into my neighbor's apartment and I told them they had to leave, the police came within moment of my calling them.
Jane S 