I would assume that they entered the park at an ordinary gate. How else would one enter the park? I would therefore assume that they paid the ordinary fee (for instance a day use fee), and were given the usual packet of information brochures, which probably included several references to the ban on biking below the rim. After all, bikes ARE allowed on the roads above the rim, so the ranger at the gate would have had no reason to stop them entering.
And I do think the sentence is fair enough. $500 is not an enormous amount, and 2 days in prison is a "lost" weekend (or perhaps more to the point, a weekend's lesson), not a lost lifetime. And the 5-year ban from all nat'l parks is probably pretty standard for infractions of national park rules. Those rules are enforced by the park ranger service, which has police authority over the entire park system. This would not be the only context in which the parks are viewed as a single, collective unit.
As to the balance between the punishment and the impact "value" of the crime: I've walked just a small stretch of those narrow, switchback trails below the rim. Not only would I not want to encounter a cyclist along those trails, and not only would I worry about a cyclist's safety, but there's also the issue of erosion. The Grand Canyon is a delicate treasure to be cared for on behalf of us all. It is already overburdened with users, and let's face it -- off road biking does add to erosion. So it's not just about their own safety. It's about the safety of others and the preservation of this delicate environment.





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