This focuses on the legal designation of terrorism. But outside of that designation, it isn't true that homegrown American groups are not considered terrorists.

This focuses on the legal designation of terrorism. But outside of that designation, it isn't true that homegrown American groups are not considered terrorists. "Eco-terrorism" is not an unknown phrase in the US, for example. The Weathermen. But I don't think I have ever heard the KKK described as a terrorist organization in any context. (For a fun exercise, try thinking of extremist groups and who they attack, then match that up with whether or not the group is considered terrorist.)