Oftentimes, when people say this, they say it because they define any violent act committed by someone from a Muslim family to be an aaspect of some "global jihad" while they are more than willing to dismiss the thousands of murders committed in America by Christians as isolated events or merely criminal acts.
Regardless, it's hard to look at the IRA, ETA, LTTE, KKK, Shining Path, FARC, UNITA, etc and say that "almost every terrorist is a Muslim".
None of them ever tried to take out American airliners or tried to target Americans in their own land. Most of them never targeted Americans anywhere else either. To equate any of these groups as an equal threat to the Islamofascist threat is disingenuous.
When was the last time any of these terrorists you mention committed any attacks of significance?
OK how about "every terrorist of significance to the USA is a Muslim?" Does that work for you?
Actually one could substitute "West" for "USA" if we're talking about the present day.
FACT: Every terrorist of signifcance to the West today is a muslim.