I think they are both at fault - but more so the tailgater. He certainly should not have been so close, but the guy in front should not have hit the brakes either.
I run 41 a lot around the fox valley an no this exact area where the accident happened. I don't know what it is about this area, but people generally speaking seem to drive slow and are unaware of people coming up from behind them. I travel a lot for work and hit many cities in a given week so have a feel of what this area is compared to other cities. 41 can be frustrating to drive - and I am on both ends of the spectrum. When pulling a trailer - I am often the guy who is doing between 65-70 mph. When going via car, if traffic is fairly open, the guy doing 8 mph over the speed limit. Tailgating is never the answer. I think a quick flash of your lights will at least make the person aware you are attempting to get around them if they are in a daze of sorts. That was not the case here - but who knows what was happening 5 miles before this. I know if I see a guy quickly approaching me from behind, I will try to kick it down a bit to pass quickly and let the other guy pass. But I have also been on 41 with a line of cars in the left lane, all doing 2-3 mph over the speed limit and no where to go, with a guy right on my bumper. All I try to do there is provide a bit of a gap between me and the guy in front of me so it is only a 2 car crash versus a 3.