Well, people do not run riot burning flags here in Europe. far from it. But the idea that it's so one of a kind, or almost like shooting a soldier point blank in his home, is much less widespread. I think there is more understanding here that you can feel "our flag is being used as a cloak for things I hate, and that everyone should abhor - war crimes, persecution, hypocrisy" - and then burning it can be a useful means to call attention to those things. The heavier the hypocrisy you're fighting, the sharper the means you may have to pick to get the message across.

Of course it matters that the USA has been more involved in war than most European countries in the past fifty years, but hey we do have armies too, so I don't think it's just that.