The guy with the rifle might be a highly trained and proficient combat marksman or he might be a drooling psychopath who has never touched a rifle before he picked up this one up yesterday. There is no way to tell, since literally anyone can do this.
Open carry is a relic from the nineteenth century, when people moved around on foot or horseback and their guns, if they had them, were necessarily in the open. It still has some relevance in rural areas, for instance, if I am out looking for feral hogs and have to cross a public road with my rifle.
These idiotic demonstrations by urban nutburgers and conspiracy dupes only hasten the day when governments will prohibit the practice entirely.
In particular it is only a matter of time before some pumped-up cop, panicked home-owner, or fellow gun humper loses it and kills one of these fools. Since their rifles are always slung during these demonstrations, the nuts are in a poor position to defend themselves if they do happen to set someone off.