I suppose violent acts committed in the promotion of a political or social agenda are going to be labeled terroristic acts; having said that I am not sure that acts committed by the mental ill should be considered terrorist acts.
Also, I do see your concern as we have seen high ranking government leaders using various strategies to implement policies that the legislative branch would not. So, could the Office of Homeland Security implement an Assault Weapons Ban if they deemed it necessary to reduce the risk of terrorist acts? I have no idea, but it does seem at least theoretically possible.
A major source of objection to a free economy is precisely that it ... gives people what they want instead of what a particular group thinks they ought to want. Underlying most arguments against the free market is a lack of belief in freedom itself.
- Milton Friedman