Well the reason you would want to sit in a jumpseat is to be closer to the action of course!
Unfortunately you cannot in the U.S. And I think the reason behind that regulation is because of the risks associated with it. While most requests by pax to sit in a jumpseat are usually made by an aviation enthusaist, there is always that risk of hijacking, hence why jumpseating is not allowed.
And it is for the same reason (hijacking) that the cockpit door is closed and locked during flight!
I know it may sound ridiculous but the risk is always there.