If the connection was routed solely inside of the United States, I would say you're right; however, the connections are routed around the world, and would require the concerted efforts of several countries to even begin to trace the origin. Even if they do monitor every packet going across a backbone provider in the U.S., TOR packets are encrypted. The NSA may well have technology to crack encryption that we don't know about, but unless you've seriously pissed somebody high up off, I doubt you have anything to worry about.