I define terrorism as someone trying to strike fear into the hearts of the general public. While his letter loosely fits that definition (because he's trying to persuade people to fear/fight the government), his actions do not. He didn't crash a plane into a building to scare anyone, with maybe the exception of the IRS/Government (for fear of "the people" revolting, but honestly I don't think they're very worried. Our country is pretty thoroughly brainwashed and pacified.)