It's the danger of traffic going at different speeds. Let's say that 90% of the traffic is going 55mph through a 35mph construction zone, and you're going 30mph just to be on the safe side; you're creating more of a risk than if you were going 55mph. People coming up behind you have to slow down, try to pass you, etc.
Now, I agree that if traffic is "flowing" at 80mph in a 55mph zone, then it would be a little far-fetched to say that you should be driving 80mph. However, I think it's safe to say that driving 65mph in such a situation would be safer than driving 55mph, because there would be less of a variance in speed between the different cars.
To sum it up, variance in speed contributes to accidents. Whether it's the fault of the grandpa driving the speed limit or everyone else driving 25 over, it's still an accident. Also, if you drive 65 when everyone's doing 80, it's not like you're going to get a ticket.