Why not? It's peaceful and it sends a message--it's no more inherently bad than standing up with a sign that says "Down with the US" or whatever--which everyone would agree is protected speech.
Yes, flag burning is designed to get a reaction, but it's designed to get a reaction in the same way as, say, a KKK rally. The message is deliberately offensive, but it doesn't mean that it's not protected speech.
Anyhow, we're done with this issue for the time being, thank God, so I guess it's a moot point. At the end of the day, though, I've just never heard an articulate defense of a flag burning amendment that at its core wasn't about punishing a certain type of offensive speech. That smacks me as a bad idea.
The flag burning amendment reminds me of De Tocqueville's analysis of the Press in "Democracy in America:"
"If anyone could point out an intermediate and yet a tenable position between the complete independence and the entire servitude of opinion, I should perhaps be inclined to adopt it, but the difficulty is to discover this intermediate position."
Indeed.