Flag burning is, in my opinion, an issue created out of the most strangest
phantasmic piece of America political
hoodoo.
To put it simply: People only burn the American flag because it annoys so many Americans. They don't know how or why it annoys them, and they don't really care. I can imagine a new Anti-American fundamentalist recruit being told about the flag burning tactic, and he'll go something like
"Seriously? I can piss off lots and lots of Americans really badly just be burning their flag? Some of them will even cry and rant and rave about it in sheer frustration? Naaaaaah! Are you sure? Get outta here! Whats the catch? I mean its a just a piece of cloth, well I was gonna go kill some American missionaries but this seems so much simpler... etc etc".
The two flags usually seen burnt on TV are the American and Israeli flags, generally, but not always, in Muslim countries. I've witnessed the British flag burnt once, but, well, you could tell the protestors heart wasn't in it really as everyone ran off to burn several American ones before the British flag had burnt out. And the British reaction here was either "Isn't that nice they think we're important enough to burn our flag!" or "Isn't that a bit silly? Someone could really get hurt with all the fire and petrol!"
True the British can be particulary phlegmatic about such things, but I'm sure if you burnt the French flag in France the worst that could happen would be getting laughed out of the room.
Flag burning is not about how patriotic a nation is, how powerful they are, or how many countries they've invaded - the whole bizarre issue is simply another very strange item from Planet Americana.