Interesting thing about this is that apparently biking is prohibited due to NPS designating Grand Canyon as "wilderness". But while they manage it that way ... it is definitely not officially designated as wilderness. See recent discussion here: viewtopic.php?f=15&t=8461
And even if it was ... the corridor trails--which is what the Sedona 5 were riding on--are not part of the designated wilderness in the canyon anyway!
Of course the national park closure is a whole other issue and can't really be defended.
A couple of weeks ago, I was hiking the Widforss Trail on the north rim. At no point does it drop below the rim. The sign at the trailhead says that bicycles are prohibited on all trails that go below the rim. But it also had a no-bikes symbol for Widforss -- despite not going below the rim. Why? It would be a great trail for biking.
There are definitely some inconsistencies in the rules and/or reasons for the rules. I'm not a big mountain biker, but I can understand the frustrations of those who are.