Do you not see the immense potential for abuse here? ie. a situation where safety or security concerns are not the primary motivation, rather political motives are?
Abuse of what, since cordoning off people doesn't restrict their free speech there's nothing to abuse. Do you really think politicians look at waving signs, pro or against? It's just a bunch of people outside the car yelling. Actually some of the first cordoning off was done purely for political reasons, Democratic National Convention 1968, they didn't want the anti-war protesters screwing up traffic and shoved them down the block. BFD, they still got to wave their signs and turn it into a riot and get beat up by the cops, everybody had a good time, nobody's speech was impinged.