According to George W. Bush - they're probably not (*).
For the most part - Palestinians has limited their attacks solely to Israel. Since Palestine isn't it's own country, it's an internal matter.
Otherwise...those nasty Americans sending money to the IRA might be in trouble. ;-)
And the USA would be guilty of harboring individuals with known ties to terrorism.
* (yes I know of the one attack outside Israel - it occurred in Egypt, iirc).