Like others, I've hiked quite a lot in the Canyon, doing rim-to-rim-to-rims about once a year (heading there again in May!), I would not want to encounter a bike on some of those sections. I believe the Arizona Trail uses the South Kaibab trail, which isn't particularly narrow, but there are sections with dropoffs, and it is used daily by mule trains. What those animals would do if they encountered bikes is completely unknown, and I doubt good. I think the punishment does fit the crime, mainly, as most of our punishments, to prevent others from following suite.