To throw in my $.02 - the reason for the tether as explained to my at a Police child safety seat inspection the tether is to prevent the top of the seat from moving forward in a crash and causing whiplash that can kill little ones. The rear facing seats would not be subject to these forces, as the child's movement would push them into the seat. Which is probably why the are rear facing to start with!

And if the cops in your area offer a safety inspection - GO - Here I thought with a Volvo and what was an expensive child seat from my oldest child I was in good shape. I found out many older child seats are considered not safe and Volvo seat belts require that H shaped belt tensioner and to be MUCH tighter then I ever thought. Also the harness straps need to be tight enough that only two adult fingers can be sqeezed in. It ended up the cops replaced my old child seat with a new one on the spot as part of their safety program (they did for others as well, I was nothing special). And it took the experts about 20 minutes to adjust properly. To do it right still takes about five minutes of grunting and pushing the child seat into the car seat to get it right.