I guess you're young. Terrorism by right-wing groups is a fairly new thing in historical terms. Terrorism in the modern sense was more or less invented by the labor movement of 1890-1920, from which it spread to other left-leaning groups. The right-wing didn't pick it up until really the 1980s, with a couple of incidents in the 1960s.
Greenpeace co-founder / early member Paul Watson has said publicly "There's nothing wrong with being a terrorist, as long as you win."
See the Haymarket Affair (bombing crowd-control police in the middle a crowd) by unionists, the Los Angeles Times bombing (unionists), The Preparedness Day Bombing (anti-war), 1920 bombing of Wall Street, 22 anti-corporatist bombings in NYC in 1940, anti-war activists like Sam Melville and Jane Alpert bombing banks, courts, and other buildings through the 1960s, Sterling Hall bombing (anti-war activists, 1970), Weather Underground bombing of the White House and other buildings (communists/ Leninist, black power), 1983 bombing of the Senate building by anti-war activists, etc.
Since then it's been picked up more by green terrorists such as Earth Liberation Front / Animal Liberation Front, while the UN has officially added Greenpeace to its list of known terrorist organizations.
As the FBI counter-terrorism chief writes:
During the past decade we have witnessed dramatic changes in the nature of the terrorist threat. In the 1990s, right-wing extremism overtook left-wing terrorism as the most dangerous domestic terrorist threat to the country. During the past several years, special interest extremism, as characterized by the Animal Liberation Front (ALF) and the Earth Liberation Front (ELF), has emerged as a serious terrorist threat.
Earth Liberation Front terrorists such as Tre Arrow, Marie Mason, and Jeff Luers favored arson, using the techniques described on the ELF/ALF web site. On the other hand, SaveThePlanetProtest.org author James J. Lee used the traditional bomb to take hostages at Discovery Networks headquarters. Daniel G. McGowan (Earth Liberation Front) used arson.