Well, perhaps it not a particularly respectful action. However, it would be a double standard to grant freedom of speech to one group and not another. But then again, it seems that that is all the world ever is anymore- ignorance and double standards. I would not immediately disregard someone as unacceptable simply because it expresses dissidence. If someone despises American principals, or even America itself, then let them speak. Innovation is never achieved via conformity.
Hate speech is not protected speech as has been ruled time and again/. Desecration of the flag should be viewed as no different than any other form of hate speech. It is an action intended to elicit shock and anger, nothing more. It differs from no other form of hate.
It's kind of a weird oxymoron... Burning the very symbol that has given you the freedom to burn it without being shot for treason. It's not biting the hand that feeds you, it's stabbing the hand that protects you. Nevertheless, stschiffman is correct in observing that the SCOTUS has ruled it to be an exercise of free speech, and therefore not illegal.
I consider it just plain appalling that despite all the brave men and women who have died fighting to make America what it is today, protesters still think they are being cool and hip by burning the American flag, a flag that symbolizes freedom and liberty.
But should it be illegal? No, it should not, because it is still freedom of speech. It's basically hate speech, but at the end of the day, it is still free speech.