@ Wozitoya
I am not sure why you are citing the wiki entry for "COPS (TV series)". This show was a documentary about the police, that's why it was called "COPS" (cops being the plural of cop). It was made only 20 years ago, whereas the etymology of the word cop is dated back to 1859, predating movies & TV.
Cop is not a derogatory slang, but it's still informal. It is not as respectful as "police officer", and in some situations it's not appropriate to use. So they're not always interchangeable. e.g. "Excuse me cop, is there a problem?" vs "Excuse me police officer, is there a problem?".
Incidentally, many people take offence at "policeman/policemen" since it's sexually discriminating. The politically correct term is "police officer".