Is it? Or do we simply not call some such organizations terrorist organizations out of politeness?

I suppose one could argue that the proper definition is "A non-state entity who commits criminal acts for the purpose of invoking terror to coerce actions from others", which will capture almost all groups that we consider to be terrorist groups, though it really depends - is a group who creates fear about the food supply for their own ends a terrorist group? I would argue yes (though one could also argue that this is equivalent to crying fire in a crowded theater, and thus a criminal act).