Not only in the US. I haven't done research regarding other countries, but Germany has the exact same problem. In fact, the biggest terrorist attack ever carried out in the current German Republic was the Oktoberfestattentat of 1980, a right-wing attack. There are way more terrorist attacks perpetrated by right-wing people than left-wing people in general, even simple violence without terrorism as a motive is carried out more often by right-wingers.