This is an area where I think the rules are appropriate. I assume that properly placarded items receive special handling when shipped. If that's the case, there is less chance of danger to the public.
Our rocket motors don't explode but they will melt through many materials including metals very quickley. What if your stuck in traffic & one of these trucks has caught fire ahead of you? And the truck had rocket motors & pesticdes & who knows what else? That's what DOT is trying to prevent.