The tax protesters arguements are specious in the extreme, since income taxes are both lawful and constitutional under the 16th Amendment. (Doesn't mean they don't suck, though.)
For him to skate like this really says a lot about the state of the DOJ. Thiat doesn't surprise me, though, since I know very few Federal prosecutors who are any good. The ones who are mostly spent many years as State and Local Prosecutors before joining the Feds.