While it is widely accepted that portions of the Rocky Mountains were once underwater, and were pushed up via orogenic processes, it is also widely accepted that the vast majority of the central United States spent a significant amount of time underwater due to a higher global sea level.

Since this trough only covers the western 1/4 or so of Montana[2], I think it would be safe to say that, on average, a shark tooth found in Montana was not found in an area of orogenic lifting, thus was likely deposited there by a shark, or something higher on the food chain than a shark, during a period in which the sea levels were much higher than they are today. Are there places in Montana where orogenic lifting created dry land? Yes. Are they as likely to be trafficked by humans as the others? No.

I suppose they could also have been recently deposited by a migratory bird.

[1] http://pubs.usgs.gov/gip/continents/

[2] http://pubs.usgs.gov/gip/continents/cretaceous.jpg