We all know racing is risky. It shouldn't be risky for the fans, but in some cases, as in rally racing, it can be. Spectators like to be as close to the action as possible, and when your course is a closed mountain road, there are very few ways to provide adequate barriers and runoff zones.
When you're watching a race, you never want to see a classic ride's total destruction via barrel roll. And when you're racing, you never want to hear, "Aw dude, he's out the windshield!"