Any masked person at a demonstration on private property should be turned away or arrested immediately. Masked people at protests on public property should be suspected of bad intentions and directly supervised.
...and charged as terrorists.
Protests are supposed to have permits and follow the rules. It's a no-brainer that people at protests shouldn't be allowed to obscure their identity. The other issue should be the issue of our suspicion that there are paid organizers and protestors. If that's the case, these aren't protests, they're theatrical events. Those are the rules they should be governed by, including wages and laws for the employees and employers. Another situation....flag burning legal? So why not enforce the fire laws?