Edit: See edit at the end. I was working on incomplete information.

Looked it up, and here's what Illinois traffic law has to say:

A driver must come to a complete stop (and yield):

• When a pedestrian is in a marked crosswalk [notably, this one is marked as a "zebra crossing"].
• On school days, when children are in close proximity to a school zone crosswalk.

So you technically don't have to stop until a regular pedestrian steps into the crosswalk, but once they decide to enter, you have to stop your shit.

Additionally:

A driver must yield:

• When a pedestrian is in an unmarked crosswalk on the driver’s side of the roadway, and there are no traffic control signals.
• When making a turn at any intersection.
• When making a lawful turn on a red light after coming to a complete stop.
• After coming to a complete stop at a stop sign or flashing red signal at an intersection.
• When a pedestrian enters a crosswalk before the traffic light changes.
• When a pedestrian walks with a green light, to a “walking person” symbol or to a “walk” signal.
• When a pedestrian leaves or enters a street or highway from an alley, building, private road, or driveway.
• When a pedestrian enters an intersection with a flashing yellow arrow.
• To a person with disabilities, including physical, hearing and visual disabilities.

Only the last person was driving illegally, but I think the other people who didn't stop were being dicks.

Edit: I dug further (i.e. into the compiled statutes and not just a handbook) and found more:

Whenever any vehicle is stopped at a marked crosswalk or at any unmarked crosswalk at an intersection to permit a pedestrian to cross the roadway, the driver of any other vehicle approaching from the rear shall not overtake and pass such stopped vehicle.

So actually, the second-to-last car was violating traffic law too. The other two could be debated given the middle car wasn't yet "stopped", but it sure was still a diiiiick move.