Do you see a realistic alternative? Bomb Boston? Respond in kind, by bankrolling or arming Al Qaeda?
For the most part, it wasn't so much a case of the federal government supporting the IRA directly, but excessive tolerance of the groups who did so - letting Gerry Adams into the US when his terrorist links should have precluded entry, letting his allies fundraise. Ironic, too, that support for the IRA was one of the very few areas where the Gaddafi regime and the US were on the same side!
(On the other hand, if a resurrected Hitler and Roosevelt both came to the UK for a reunion, it would be Hitler who got the friendly treatment and fast channel at immigration...)