Our "founding fathers" were considered traitors and once they signed the Declaration Of Independence, King George issued death sentences for them.   Though they were engaged in a violent revolution (because the British would not let the colonies secede without a fight), but "terrorism" is a term used for acts specifically aimed at instilling great fear in a group.   
As Uncle Dave put it "the creation of widespread intimidation by imposition of senseless and sensational violence. The idea is essentially a propagandistic one, rather than a strategic act."

So I wouldn't call the Boston Tea Party or releasing lab animals from cages or even most cases of assassination of political leaders "terrorism."   "Terrorism" applies when a larger group is being intimidated -- violence is feared and its targets unpredictable except for being people in some category (e.g., Jews, blacks, Catholics, gays, Wall Street Execs, clinics that offer abortion services).   
I guess you could argue that there can be "economic terrorism" when a group aims to ruin a group of people financially, but I think that may be stretching a point.   Some have tried to label product or company boycotts "economic terrorism," but I think that blurs some important distinctions.

The suspect in Tiller's murder was tied to Operation Rescue and maybe other extreme anti-abortion groups and reportedly had bomb-making equipment in his home too. I'd say that the term "terrorist" fits him a lot more than it does people who get between whaling ships and whales or perch in old trees to keep them from being cut down.