The SPC probably operate in some sort of "special ops" capacity. They're all very skilled fighters, and as such, they would be wasted if they were used as nothing more than imperial guards. Notice that Little Tokyo doesn't appear to have a police presence--only the Cats ever respond to Ninja Crow and robot attacks.
Aw, that makes a lot of sense. Thankyou. That's quite a responsibility. Do you the citicens of Tokyo are so good that they don't need a police department for like bank robberies and traffic violations, or would the narrator tell me that the writers didn't think showing that part would be entertaining? or something else entirely?