Steve … there are concerns like that for college police departments – see stories on campuses who don’t report crimes they’re supposed to, or whose “staff kids” get a pass if caught, or those campus PD’s whose officers joke that they’d lose their jobs if they ever arrested a student (staff member, administrator, whatever.)
The things that make it work are, first, the officers take the same oath as everybody else, and those offenders caught go through the same court process (jail, bond/bail, hearings, trial, and sentencing) as everybody else, with the officer making the same kind of complaint and sworn statement that a municipal, federal, or state officer might make (with the same penalties for perjury, fraud, etc.) And, for the most part, proper law enforcement IS in the company’s interest, even where it involves dealing with individuals or selected groups of people within it in a way that might be an embarassment (embezzling comes to mind, but there are other examples…) when investigated.
And, as I’m sure you know, your concerns hold true for governmentally based police as well. What happens to the constable of East Overshoe who arrests the mayor’s kid? He “guards” the dump for three weeks, and is eventually dismissed for unshiny shoes or something … and yeah, I have a specific locale and officer in mind for that fictionalized example, which is not unique.
You have a good point — private entities are frequently less trustworthy than governments whose taxpayers make up their pay base, but … not always, and most of the time it actually works pretty well where it works, and becomes a parallel for the activities of transit police which ARE government entities because the railroad is an arm of some government or other (note that this applies to Amtrak, which IS a Federal agency.)
Finally, remember that having your own police doesn’t mean that the local, state, federal Law Enforcement agencies around you don’t also have jurisdiction on your property, and provide some accountability to these private police agencies in the same way that they do to local, municipal, county, and state agencies where a major incident or questionable situation is involved, for example, in many places, the State Police (or SHP) investigates officer involved shootings when they occur, not because the city police aren’t trustworthy, but to avoid any appearance of exactly what you’re talking about.