I don't think you can conclude that they only target american airlines. If anything, terrorists are oportunistic. They happened to choose american airlines, as there was no other option for them to get the timing of the events down. Much easier to find four or more of the thousands of domestic flights that are near your 'targets' rather than trying to get flights based from overseas synchronized.
For all we know, the four airliners we know about weren't the only ones they had targeted either. The terrorists didn't plan on us grounding all air traffic so quickly and diverting all the international traffic.
If I remember correctly, didn't they find box cutters in the trash from at least one plane that had been grounded?
Gamma