Actually, M4s and other rifle caliber carbines are good police tools. They are more accurate than pistols and shotguns, and are handy, intuitive weapons to carry.

They don't over-penetrate any worse than other weapons, as well. Any round capable of killing a human being will over-penetrate, but in fact, pistol rounds over-penetrate worse than 5.56.

What people get hung up on is the way they "look". While an M94 Winchester carbine would "look nicer", while retaining roughly equivalent capabilities, the M4 carbine is more durable and reliable, and has spare part, training and accessory supply chains that are unsurpassed.