There should not be a ban on snowmobiles in United States nation parks because they do not have any harmful effects on bison migration. It would be very easy to restrict access to the specific areas that bison migrate with out having to ban snowmobiles throughout the whole national park.
Unless snowmobiles kill large herds of bison, migration patterns shouldn't be affected. Bison are huge creatures capable of destroying snowmobiles. Loud noises produced by the vehicles might scare the animals, but snowmobile trails should have little to no effects on bison migration patterns so long as the herds don't use those trails for their migration. Don't ban snowmobiles--simply limit their use.