American's seem inculcated into the norm of owning or carrying guns. Over here in the UK if I knew someone was carrying a gun I would inform the police.
But not because it was against the law but because carrying a gun is aberrant behaviour (in the UK). You would have to be mentally disturbed to carry one on the street.
And sometimes even when we (Americans) suspect a gun owner of not being quite right in the head, we don't turn them in.
I have a neighbor who walks the neighborhood, mows the lawn, and works on his vehicle with two pistols strapped to his waist and a huge knife. A couple of weeks ago, in addition to two pistols, he also had some assault looking weapon on his back while he stood on his lawn surveying the neighborhood. I honestly don't even know if that's legal, but when you live in the rural American West, guns are part of everyday culture. As far as I know, no one calls to complain about him.