Sworn police operate under the color of public authority. They may constrain a citizen’s personal freedom largely at their own discretion, and they are granted wide latitude in exercising the state’s monopoly over the legitimate use of violence. Allowing moonlighting cops takes policing into the marketplace, and it transgresses the boundaries between the public and the private. Three practices of moonlighting police officers are noteworthy: the use of public police officers to enforce privately established rules; the enforcement of public traffic, parking, littering, and loitering ordinances in private spaces; and the possibility/reality of using public funds to underwrite/subsidize police forces that are under private control.
I would never make a recommendation in favor of moonlighting.
Similarly, the use of publicly-owned and -marked vehicles for private enterprise is, in my opinion, an illegal gift of public funds.