You don’t have to look far to find actual concrete examples of violent leftists, and leftist home-grown terrorists. Look at almost any leftist demonstration (anti-war, pro-abortion, whatever), and you see a cauldron of violence waiting for the slightest trigger to erupt. And leftist terrorists have been active since long before 9-11.
OTOH, the “warning” about right-wing groups is all hypothetical, and there isn’t a single concrete example of a right-wing terrorist, nor of right-wing groups erupting into spontaneous violence.
See my reply to GeronL