[dragonwriter]

The same people who would do so for an incitement to immediate violence, intimidation, or terrorism (which are generally illegal under other criminal laws alreadyn whether or not destroying a flag is involved): prosecutors, judges (trial and appellate), and juries.

Can these actors be unfair and express biases not in the text of the law? Sure. But that's more an argument for policies to address that problem than it is an argument against additional criminalization of a subset of a category of generally already-criminal acts. (And against which, I should add, there are plenty of other arguments.)