It is a hard one with the truck.
The driver is a complete idiot, but the truck seemed to bash the horn rather than emergency brake. I would fine both if it was me.
The driver of the car should get negligent driving charge. You do not have the right to brake anywhere you like in the traffic- unless it is a emergency. You must take care that your actions do not endanger others- even if you are in front. Just like it you have no right to go into a intersection when the light is green- you must do so safely and only when it is safe to do so.
The vehicle in front does not have carte blanche to do anything they like, nor can they just decide to brake and stop in a lane unless it is for emergency reasons or because the traffic ahead has stopped. Just like you can't just decide to enter a intersection and then stop and read a map etc.
If it went to court esp. if someone is injured and compensation was sought, then I would expect both drivers would be found to have contributed to the accident. Some states may see it different but that is the general principle.
As far as multiple rear end crashes- in NSW the last car to hit only pays for the car they hit and so on down the line. I know because this happened to me 20yrs ago. The police that arrived were fortunately- the accident investigation team, lucky us. The fool at front slammed on the brakes on a major road for no reason and we all barreled in. The cop stated that as no one was injured and cars were movable (to a side st) she would not charge anyone. But the first driver- the idiot, wanted everyone fined- the nice officer obliged. The idiot was charged with negligent driving - they had no lawful reason to emergency brake, and by the very nature of it been a arterial road, the whole road is a NO stopping zone. Just like you can't park your car on a freeway lane and have a picnic.
A similar situation is when swerving to avoid wildlife, if you cause a accident- you are at fault. You can not drive in a manner that endangers others- no matter what signs, lights, wildlife etc.
I agree though that many cops have very little law knowledge and just rely on us copping it. Or they pick the easiest target and don't want to get involved.