Well, first of all, for every car that is rear ended, you have another car that is doing the rear ending. The rear ending car is having a FRONTAL crash, so at best the numbers would be equal.
Being rear ended while rear facing is also not the same as a frontal crash forward facing. In a frontal crash, there is momentum, so the child moves forward into the harness and the harness is pulling them back. When you are rear ended with a rear facing car seat, the vehicle and car seat are actually getting pushed out from under the child and the harness is pulling the child along against inertia. Most of the time, the car is still moving forward, and the crash rolls the car forward so there is actually some forward momentum and the child is already being pushed back into the seat and doesn't need as much of a come-along pull. Then you have the rebound of the rfing seat (even a tethered or other rebound controlled seat rebounds, just, with more control) which keeps the seat moving with the child and the end result is much less force than a frontal ffing crash.