Mos Def Arrested

This past Friday, Mos Def was arrested while giving an impromptu performance from his Katrina album at the VMA Awards. You can watch what went down here. I think this begs the question, what purpose do police forces serve in this country. Are they supposed to play in politics(with political arrests,beating up people, assassinations--i.e. Fred Hampton) or are they supposed to serve the people?