The rubber bars channel water away from the trail to avoid damage. When the trail is dry and ready for use again, the bars fold down as the biker rolls over, and pop back up when he has past. Many of the misconceptions about bikers are not caused by what I call true mountain bikers. Sadly, in the Southern and Northern Kettle Moraine forest, damage to trails does exist. On trials used primarily by bikers, if there is any erosion at all, it is on the upward side of hills.
Many parents take their kids out on the trails for a day of riding.
I am certainly in favor of exposing new riders to the sport, however most families know nothing about biking and ride inferior department store bikes. A biker who is in control of a high quality machine can easily ride over a trail at 30mph with not a trace left of where they have been. However an inexperienced rider skidding down a hill at 5 or 10 mph on a cheap department store bike can cause substantial damage. All bikes are not created equal. Riders must be aware of their limits, and of the limits of their bikes.