The study referenced by Iggy specifically states that it has in mind the possibility of having helmets cope better with such impacts, i.e. not "helmets are killing us!!!"
If the helmet protects the entire head, and is fastened securely to the head, then a severe enough impact is going to affect the area where the head attaches to the rest of the body. That would be the base of the skull, or the neck. The only way to change that would be to attach the helmet to the body, the way a HANS device does, but the HANS device is not intended to carry impact forces from the head to the body, but rather to limit the motion of an otherwise unrestrained appendage. A racing driver rarely actually hits his head on anything, but rather his head is tossed about by the impacts to his vehicle, with neck injury the most obvious resulting possibility.
As motorcyclists existing HANS devices are too restrictive. Sitting out in the airstream atop our vehicle, we need more range of motion in all three axes than a HANS device would allow. You have to raise the head if you're down over the tank, you have to look over (or under) your shoulder to change lanes, you have to look down at your foot when you stop. To brace head impacts with the body we'd need something like a HANS device, but with inertia reels on the attachments. But what happens if those attaching cables catch on something as we slide merrily alongside our failed steed?