I am inclined to believe a recent discussion that argues police unions are the root of all this. I don't think you can say it's anything fundamental about law enforcement in the United States, as sheriffs and state troopers (highway patrol) don't seem to have the same scale of problems. You might argue it's an urban vs rural thing, but they still deal with their share of violence, arms, drug traffic, etc. Sheriffs are elected, and there is no sheriff's union.