Well, for one, desecrating a grave IS against the law, while burning a flag is not.

While I personally would not go out and burn a flag in protest because I am disturbed by the image of burning anything in protest, I do support the right to do so. This was a nation founded by those seeking freedom to express themselves. If you want to do so by burning a flag or your bra, as long as you don't bring harm to others, then I feel that I have to support your right to do so.