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.