In the scene, John Voigt is pretending to be fishing near a nighttime event hosting the President. Naturally, some cops or federal agents show up and ask him to leave.

John Voigt responds by asking the agent if he is aware of some obscure State fishing law that allows anyone to fish anywhere in public waters.

The agent responds by asking John Voigt if he is aware the agent can detain and hold him for up to 48 hours without cause.

This is the point where the audience laughs, and the scene was a perfect windup to that punchline.

But the "psychological warfare" aspect comes into play when you consider exactly WHAT the audience was laughing at. The audience was just told that they have no rights. A guy in a costume can arrest you without needung a reason, and he doesn't have to let you go for 48 hours. The Revolutionary War was started for offenses less grievous than this!

By laughing at this scene, the audience is trained to dismiss this abuse of power as a joke. The audience has just been told that costumed guys with badges can essentially kidnap you even if you haven't commited a crime - John Voigt made it clear his character was completely within the bounds of the law. The agents were the ones breaking the law in this scene, and yet no one threatened to arrest them.

By turning this into a joke, the audience learns to accept it and laugh at it - instead of calling up every representative in their city and state to demand answers for why this is allowed to happen.