The driver was probably driving for a long time on that stretch at probably 70 mph or so, and suddenly came across some people who thought it was be cool to ride their bicycles at 15mph out there. Deadly stupid choice.
So what. Wouldn't you slow down if you came upon a bunch of people on bicycles on a highway? I agree that it was dumb of them, and of the driver of the vehicle that should have been *trailing* them with emergency lights on.