Motorcycles usually don't need to be as precise with line choice, the machine (and thus system) is heavier and needs more fore-aft stiffness, weight is not as big an issue so larger stanchions can be used, etc. Still, people riding upside down forks don't seem to have much of a problem with torsional stiffness. At least in most cases. BTW, all of this has been discussed over multiple pages before.