To a certain degree it's because of the gun culture in the US. But I support gun rights, enjoy sport shooting, and also would not immediately think "gun". My hypothesis is that police work in the States tends to attract the kind of people who want to be macho and point guns at people but lack the cajones to join the military and actually have those people want to kill them back. This obviously doesn't describe all or even most officers, but I've met enough kids like that in my high school years who dreamed of heading to the police academy that I believe it's significant.