I'd love to know how cars suddenly got involved when rolling down some sweet single track on a mountain, doing tricks in a BMX set, or when racing fellow cyclists through a closed course.

Bicycle helmets are also valuable in bike-on-bike and single vehicle crashes.

... did you even watch the video? you can be miles away from the nearest car and still benefit from a helmet. yes, cars are dangerous and we can better multi modal infrastructure, but your comment doesn't apply here at all