They already have. They did that in seattle.
But this raises one important observation and one important question:
1.) If rioters arm themselves then that is their right as American citizens and I for one would not deny them that right.
2.) If they do then those who support the riots would be faced with an uncomfortable question: Do I support those who arm themselves so as to loot, burn, destroy and otherwise break the law with impunity or do I support the ones who arm themselves to prevent it?
You seem to have lost it once again. Protesting isn't only legal, it's a right. Rioting is not legal and stomps all over other's rights. Of a Protester is armed (depending on the laws) then he's not breaking any laws. If a Rioter is armed, that raises the illegality to a higher level.