It's a biased sample. American police forces attract gun enthusiasts who want to carry and use guns on people. Go to a police forum and see how often they share and gush about weaponry and moan about what their jurisdiction won't give them. It's like kids in a playground. These are people who write "you're fucked" on a gun they use to kill an unarmed person.
Most gun deaths are by handguns, not "assault rifles". They also know that a federal ban can be negated by 2 minutes of work with a drill bit.
Similarly, police aren't afraid of people pulling assault rifles out of their glove boxes and pockets.