Yeah and I would argue it is actually pretty unnecessarily dangerous to do so both for the riders and wildlife.
Exhibit: A) that mountain biker who ran full tilt into a bear with cubs and got killed.
I know I have been put into dangerous situations plenty of times by mountain bikers while walking on trails that are hurtling around blind corners in areas where walkers or even horses are common.
I am not saying bicycles should have an air horn on them, but like bells that jangle loudly or like anything really...