Your decision to sanction mountain biking on trails is the stupidest thing the Club has done since it allowed Glen Canyon to be dammed. This is nonsense.
Homo sapiens is by far the most selfish of all species.
Now we want to be able to go wherever we want in whatever manner we want. We aren't happy bicycling our hundreds of thousands of miles of existing roads. We want a yet higher level of titillation. This is not just a Sierra Club problem. Worldwide, humans are driving dozens of species to extinction every day.