"The longshoremen were apparently there to work, but were not about to confront the protestors, as any aggressive action taken by them would be known by a well-known legal term: "riot". Were the police trying to encourage them to do so?"
Maybe you are right maybe you are wrong, it wasn't clear in the article:
"The longshoremen were standing on the side of the road and were not involved in the protest."
The longshoremen were not involved in the protest, but this does not mean they did not interact in anyway with the protestors. It *mostly* means the longshoremen were not members of the protest. For instance, if the longshoreman *had* started beating the protestors with clubs, you *could* say they were part of the protest [or acting against the protest].