Why do the police have AR-15's?
From what I can figure, the police don't want to lose the arms race against the drug distribution network, or against anybody else either.
It hardly makes sense to send the police out to police people who are armed far beyond what they themselves have at hand. I can imagine the police departments themselves were the ones demanding more powerful arms.
The way to prove or disprove this would be to find out who had what arm first, and whether the police were reacting to the environment in which they worked, or were ahead of the game.
Dman: Have you read C.J. Chivers book The Gun? It's a military history focused on Stalin's Kalashnikov, its precursors, and the changes it's brought to warfare around the world in the last 66 years. I read it when it first came out, it came up in a discussion here a couple weeks ago, and I then checked it out for a re-read. It's even more interesting the second time through. The author is a demon for research detail, and has a background that gives him standing to state some strong opinions. I can't imagine you haven't read it, but if you haven't I think you'd enjoy it.