Of course, it’s more than the cops letting them block the bridge, they made them block it by blocking them off at both ends.
When did it actually describe anything remotely like that?
The hypothetical situation being referenced here is very similar to a real thing that was widely publicized a few years before the time of publication of the comic during the height of the Occupy movement. Whereas the details here are sparse, during the Occupy protests police officers were actually reported (by the protesters at least) to have effectively sheparded the crowds onto the bridge, where they proceeded to blockade them and then arrest them for blocking traffic. Many people were unaware that the police had, in fact, directed people onto the bridge, and believed that the protesters were blocking the bridge in protest and thus had committed a crime, and criticized the protesters for proclaiming their innocence and that they were entrapped.