Chelsea Manning, back when she was Bradley, observed that the best defense against surveillance is to have a low profile. After all, they're listening to everything and recording everything. But they're not analyzing everything. The NSA's approach is now and has always been to increase the likelihood of finding a needle in a haystack by making the haystack as large as possible. Firing up a TOR node is basically like lighting a match next to your needle. Suddenly there's smoke, burning embers, crackling, etc. Granted - the NSA may or may not (go with may) be able to read what's coming out of and going into that TOR node... but all of a sudden, they have a reason to care. Intelligence is about profiling, probability and surveillance. If you match the profile of someone they're interested in, there's a probability you will experience surveillance.