Whether or not someone is breaking the law is a very difficult question. A cop must figure out what a person is doing and who they are. Is the person in the middle of a mental health crisis? Are they subject to special rules such as having been previously trespassed from a location, or are they in violation of a court order? Is this person stalking someone? When first amendment-protected "speech" become non-protected shouting in breach of the peace depends largely on context. What type of property are they sanding on? Are there special rules at play, such as publicly-accessible private spaces? (bathrooms, some areas of hospitals etc.) Even then the cop must decide if an arrest will actually prevent further issues, or whether some sort of non-arrest option might be better.