In most American areas, when you see a uniformed police officer there is actually a good chance that he (or she, I guess) is not actually working for the police department. It is possible that they are working a "detail," hired by a local company to do a certain law enforcement-like task. There are a few things wrong with this right off the bat:

1. Police are supposed to be paid (from tax money) to enforce the law whenever the law needs enforcing.
2. They are supposed to do so equally, and for everyone.
3. They are not supposed to take bribes.

These details usually involve working to catch shoplifters or keep striking workers in line. It used to be that companies had to hire professional goons for these jobs (that's how the Pinkerton security company was started). Now, police officers are hired for the job, and this amounts to accepting a bribe from one entity, and necessarily exhibiting a prejudice in favor of the paying party. In normal circumstances the paying party is the taxpayer, and we (ideally) can expect the cop to be partial only to the community in which (s)he works. Even if the cop is an emotional super-human and can work objectively under these circumstances, at the very least it amounts to demanding additional money to do the job you're supposed to do. Oh well, I guess I'm just bitter because I can't get away with that.