Well I know man, I think it's a semi-bullshit law as well. All I'm trying to say is that when there's a large hunk of metal behind you car (ie, unless you drive with your eyes glued to the rear view mirror) you're not looking at it. That isn't a problem as such, but when you brake suddenly and there's some knob tailgating you (yes their problem I know), that hunk of metal is going right through their windscreen/helmet right at head height.I have complete control of where my bike rack goes........ it follows the car. Having bikes mounted to it doesn't change that.
I can also see it when reversing just as well as you can see the rear bumper or boot of most modern/sloping sedans.
With bikes on the rack, it's a lot more obvious and visible for the people following you - even though they should be aware anyway, and shouldn't be tailgating.
Put there in place to guard against the lowest common denominator...