That line of thinking only works if social order is maintained and the police come to intervene in the illegal road block. As we've seen in the last few years, that almost never happens and the police actively let the protestors block roads illegally. At this point, it's probably being argued as a "free speech" issue rather than a safety/law one. I.e. that "protestors" have the "right" to "peacefully assemble" on a road.