Interesting that the 2009 Virginia Terrorism Threat Assessment also includes reference to "sovereign citizens movements".
http://wikileaks.org/wiki/2009_Virginia_Terrorism_Threat_Assessment%2C_Mar_2009
Clearly these movements are are being categorised that way. Whether this is accepted as a convenient shorthand or an attempt to damn by association will be no doubt coloured by the politics of each individual.
Whether the use of the word is accidental or more sinisterly motivated, its use is, in my view, inappropriate. A more accurate word might be rebel, subversive, dissident, or if arms were involved insurgent. All of these words fully convey the sense of opposition to the status quo and current power and authority structures.
The word "terrorist" suggests to me dramatic and deadly injury to civilians or non-belligerents ( for political purposes)
I can't remember where I came across this cynical observation, but here it is:
Question: Whats the difference between an army of occupation and a terrorist organisation?
Answer: An Air Force