- The job of three-letters agencies is to find out all various enemies of the state. That includes terrorists, but also various gangsters, officials of sort-of-enemy states (like Russia). That's why they exist.
- Following US nation interests has higher priority than rights of citizens, especially in other countries.
- People, that US have a reason to spy on, probably will use Tor/Tails, or at least try to find something about it. It makes sense for NSA to filter those people and focus their spying on them specially.
- Not all Tor interested folks will be evil, but the percentage there will be much higher than in just random internet. So it makes sense to focus on them. Just like it makes sense for a local police to be in a neighbourhood, that's known for a higher criminality.
So I understand why NSA does this, and why do they single out Tor-interested folks.