Are halloween costumes supposed to be funny? Some are, but I don't think it is a requirement. They can be cute, novel, clever, thought-provoking, whimsical... or funny. I think the twin towers were aiming at the 'novel' category. I don't see any reason to think they intended it to be funny. I also think it is a bit ridiculous to be offended by it. If you want to be offended about something regarding 9/11, how about the large number of people who have made money and/or careers based on exploiting it? That seems like better fodder for being offended than a couple of college girls dressing up for halloween. barfo
As despicable a human being as I think Bill Maher is, his Steve Irwin costume has to be considered one of the funniest ever.