Cop is an acronym. You can pronoun it. It's okay to pluralize acronyms. RBI is not. To prove this minor point, people say ribbies to make the abbreviation, RBI, into an acronym. Other acronyms are scuba, radar, etc.
It's only a minor point, but RBI doesn't really get an "s" on the end to make it plural. It's not an acronym.
I wasn't an English major, but I was a journalism major.
Actually, if I'm not mistaken, cop comes from the term "copper" used for police way back in the day. Copper was due to the fact that most badges were made of copper at the time and was later shortened to cop.
Edit:
I'm wrong on that. Cop comes from the verb form of cop: to take or seize. In 1844, the first reference to cop taking the meaning of "to arrest".
http://www.snopes.com/language/acronyms/cop.asp
