Ben, 

If, as you note, the police aren't acting in response to criminal behavior occurring right in front of them, what -exactly- do you expect the property owner to do? If they are actually on the private property, as opposed to the sidewalk nearby, the owner's sole legal recourse is pretty much "call the police and have them trespassed." If the same ones return, call the police again and possibly have them arrested. Given reality, they are at worst going to be cited and kicked loose, as they'll claim indigence in the face of a fine and no one wants them cluttering up the jail.  

Yet how quickly would those same police, pushed by the bleeding hearts, ACLU, and our federal DOJ most likely, be to act to arrest, charge, and sue penniless a property owner who did anything more aggressive than the above?

So, again, on what rational or just basis could we hold dis-empowered property owners responsible for behavior they are not committing nor have any way to deter, prevent, nor control?