Who is at fault is a gray area but that is why you should always leave a safe distance between you and the car in front. When I was rearended, my car's bumper stopped about a frog's hair from the car in front of me. CHP officer was complimentary that in rush hour traffic I'd left that much space so that I wasn't also cited.

I've used a hitch rack for years and heard very few horror stories about bikes getting smashed that way -- and tons and tons of stories about the owners themselves driving bikes into roofs.