Warbler said:

can we really conclude that Al Qaeda would only go after political targets?

Why waste good money and explosive on targets that don't interest them? They are a political organization, they go where the money is. They hit the WTC because, guess what, there was money there, World TRADE CENTER! It was a huge kick in the nuts not just to american economy, but to world economy!

That they wouldn't go after targets where there are many Americans gathered together for them to murder?

If you're going by that reasoning they should've done that during the Super Bowl. Juiciest occasion right there!!
I don't think they're concerned with just killing people for the heck of it. If you really think that the baddies want you dead just because they're bad, I'm sorry but you're naive.