Interesting case.
Is a flag property or is it owned by the public at large as a symbol of our nation?
If a person buys an American flag from a store, it's theirs to do with as they wish.
A flag is personal property, like a dog. Stopping punks from trampling the flag that is their personal property is like stopping punks from kicking and beating their personally owned dog.
Even if you are violating the law by interfering with a person and their private property, there are times when you are morally justified doing so. This was one of those times.
You buy it, it's yours. I buy it, it's mine. Do with yours as you please.