That's true if and only if the SUV is moving at a high relative speed. Otherwise an SUV will not out-inertia the Earth or anything bolted to it, like a concrete

That's true if and only if the SUV is moving at a high relative speed. Otherwise an SUV will not out-inertia the Earth or anything bolted to it, like a concrete paver, a brick wall, or a lamppost. And yes, if the car hits you at a high speed, you're no better off than a pedestrian and the helmet is unlikely to help. But without a helmet, it really doesn't require much energy for a head strike to be a fatal or life-changing injury. If riders face a baseline risk from non-car collisions, there's a strong case to be made for mandatory helmets, even if it means the risk from car collisions increase.