This website has some basic information. I do not know the law in either of your states of interest. http://www.safekids.org. You should consider that the reason behind the laws is passenger safety. In general, people, in particular, children, can be at risk in the front seat of a car with an airbag. This can mean that a 4 foot 10 inch petite women may have different risks than a 5 foot ten 145 pound person. There are a lot of other factors that play into passenger safety - but, two - height and weight have been given a more heighten status. Even an adult that is really short and really light may be at a slightly higher risk. If the website does not address your question, someone more educated on the topics than me will hopefully post a comment to assist you. Good luck.
You can seek this information from attorneys from Idaho and Montana or just do a search on the interenet and find this out too. Additionally, the Idaho/Montana state parole would probably have this info too.