I have yet to hear of a mountain biker causing a collapse that destroys a nearby road, which someone raised as absolutely a possibility here. Less that the risks are "novel" and more that they are absolutely known in the mining industry which is why this is illegal.
To borrow a metaphor, I'd say that now we are just quibbling over price. :) Discussions about relative risk, externalities and the like are all very relevant when it comes to other hobbies as well.
It's just that most of the relevant conversations surrounding these things were had decades ago with other hobbies, and many hobbies probably wouldn't have a prayer of actually passing muster if they were new today - Full contact football being a good example.
This issue of legality that you raise is a good one though, and likely the most relevant, unless this guy is going out into the desert to do this.
Aside from the obvious personal risks of mountain biking (falling, personal injury) some usually unauthorized mountain bike trails can absolutely do damage to forests and ecosystems, based on bike traffic an erosion. I haven't heard about destroying a road, but there doesn't seem to be a very good reason to pick that as the specific line for acceptability. I don't think destroying roads is generally acceptable, but we also don't know if there's an actual risk of that happening in this case.
Mountain biking is well known for being an unwanted presence. It makes hiking trails into miserable obstacle courses. Made worse by the fact that if it's allowed at all, people will go as fast as they want, it's not like there's radar traps set up.
I'm in favor of majorly reducing all hobbies that have danger or inconvenience to others. If you can't play 110DB music in the streets at night, there's clearly a general precedent and consensus that your hobby is not anything special and has no right to be a public nuisance or environmental disaster.
