Of course the DOT agrees, but they kind of have to. It's not as though a DOT spokesman could say, "It's bloody insane to think you can go exactly 55 on 285, and the law that says you have to is meant strictly to let us increase revenue and give us an excuse for pulling over anyone we think is suspicious."
The DOT could argue that their behavior was unsafe because, had an emergency vehicle needed to get somewhere at faster than 55mph, their actions would likely have prevented it from doing so (the emergency vehicle could have gotten stuck in traffic far enough back that the obstructors would have been unable to see it and yield).
Of course, another issue that needs to be considered is that if a certain number of cars per minute are entering a road, stopping distance will be proportional to speed minus a constant. Thus, under some traffic conditons, 60mph may be safer than 55mph.