Sorry, wrong guy. I'm not American but I'm assuming America has some aircraft that perform some sort of air defence.
You need to read the transcripts of the various air controllers who were trying to coordinate air defense on the morning of September 11. There was a certain amount of confusion, but the main problem was that they didn't have enough time to get planes in the air and fly them close enough to shoot down the airliners. On the other hand, Russian bombers are tracked even before they leave Russian airspace. If any of them decide to come to America for a visit, we will have had plenty of time to be ready for them.
After the attacks, there were fighters flying over New York and other cities 24 hours a day. That's the only way to prevent a similar attack. It just is not possible to get a fighter off the ground and have it shoot down a passenger jet in time.