Actually, it isn't. Treason is the only crime actually defined in the Constitution (Article III, Section 3), and the definition is very specific:
Originally Posted by Lanie
Treason against the United States, shall consist only in levying War against them, or in adhering to their Enemies, giving them Aid and Comfort. No Person shall be convicted of Treason unless on the Testimony of two Witnesses to the same overt Act, or on Confession in open Court.
The Congress shall have Power to declare the Punishment of Treason, but no Attainder of Treason shall work Corruption of Blood, or Forfeiture except during the Life of the Person attainted.
Taking up arms against federal agents may be construed as sedition, assault, resisting arrest or any number of other crimes, but it is not treason.
And, let us remember that federal authority is derived from the Constitution, which specifies that We, the People of the United States... Ordain and establish the Constitution. In other words, if the federal government attempts to subvert the Constitution, it is subverting the document that establishes its authority, and therefore is subverting its authority. You cannot have it both ways. Either the government abides by the Constitution, or it abrogates its authority under the Constitution.