We don't have the right to block traffic to promote our political opinions.
When protestors block traffic, if the city decides they have to go, and they won't - in those situations, it's the protesters who are 'instigating', not the police.
NRA 2cnd amendment protestors, pro/anti-abortion protestors, anti-capitalist - whatever we want it doesn't matter. If the protest violates the local or regional ordinance, and the city asks protesters to move (and they do in many cases allow the protesters to stay) - they have to move, if they don't move, it's not the police's fault that they are literally required by us, the community, to move people.
Edit: I would like to invite anyone to define exactly under what conditions people think they have the right to stop traffic at major intersections for hours on end, other than of course signalling to the city beforehand.