Sorry if I wasn't clear, but this is exactly my point. Most of your traffic as an American will stay within the country's borders because most of the services you access are in the US. By using TOR, your traffic will now appear to come from an exit node that has a greater than zero probability of being outside the US. The average American user thus has increased the likelihood of their data being analyzed by the NSA by using TOR. Under great-grandparent's particular threat model, the user is worse off.

Also, we can assume that the NSA operates within those bounds because that's what Snowden's leaked documents say in describing their systems. We have their internal documentation as proof.