If you want to burn a flag that you bought, go ahead, it's free speech. Don't go around burning other people's flags because that is a crime.
I haven't burned an American flag, but I think if someone wants to burn their own, it's a form of protest. I'm not sure what you're protesting since you didn't say anything.
Oh, it's probably protesting capitalism.
Didn't expect that.┐(´ー`)┌