In most situations when someone is a total dick, it's either because their position is sketchy and they're attempting to scare people away from the argument, or because they are socially retarded and lack the necessary skills to form their arguments into something socially acceptable.
I've never met Linus, but I can see how confident he is, so lets assume, like so many great geeks, it's the latter.
If your position is strongly grounded, being nice doesn't weaken your position at all, and can in fact strengthen it. Either your enemy eventually walks away feeling that they were defeated by a person with superior intellect AND manners, or in a moment of weakness they will explode in a desperate fit of rage as they refuse to see your side of things, which will cause most observers to side with you.
I'm not sure why he'd think this is an american thing, since people in his part of the world are MUCH nicer than your average resident of the USA, especially in an argument or debate.
I think he needs to get out more.