Both sides of a coin
1 - skywalker - In terms of mainland casualties inflicted, long term disruption, or 'terrorising effect' the IRA was not in the same league as the 9/11 attacks. For much of the time they sought to create low grade annoyance to inspire political change. 9/11 was much more about maximising body count. Furthermore, with a deliverable political objective you could also see a way out of the Irish mess. Much less clear in the current situation.
2 - SNS3 - While the devastation to GA in the aftermath of 9/11 was clear, the US authorities have generally adjusted their position to a more sensible one. When an Islamo nutter used a training aircraft in a suicide attack in Florida, (which demonstrated how poor light GA is at this kind of attack) there doesn't seem to have been any significant change in policy. We need to worry about things that really could deliver another 'spectacular' not about hypothetical risks that some media intern dreams up.