Okay, by your rules, if I don’t like something you say, I can go onto your property and vandalize it.
A person, through the use of flags, created a symbol of domination of one sovereign over another. Just as if the sign had been erected stating 'This is Mexico's land, not US land.'
So, aside, as I already conceded, that there is the liability on the vet's part for the value of the property taken (five bucks), which has been offset (through publicity of a commercial enterprise), what's your problem?
You have a right to yell Fire in a crowded theater. You're also not immune to prosecution for the results of your action.