That's actually the crux of the problem: there are no dedicated mountain bike trails on public land in Marin. So people who want to mountain bike use the fire roads, and often skip over to the 'single track' where bikes are disallowed. (For the record, I've personally never done this... but I'm also not a 'real' mountain biker.)

And of course, there are irresponsible mountain bikers out there who take risks, scare hikers, and deserve tickets. And those few bad actors lead to rules like this one... I think.

Really depends on the trail.