The Russian are in the process of arming the border now. Initially they are deploying intermediate range nuclear systems with a range of all of NATO Europe. The Finns were not in this peril before joining NATO and NATO is supposed to let them fend for themselves? I honestly don’t think you really believe that.
You are wrong that Americans have to man the borders of NATO countries that border Russia, we won’t be manning Finland’s borders if we accept their request to join.