I think it is more likely Americans can only stand 90 minutes of average or bad movies. It's not like a lot of people were leaving Batman or Lord of the Rings movies early. Give us a good long movie and people will watch it. The problem is we get too many bad long movies so people start to think long movies are just bad in general.
I don't have a problem with long movies (if the length is needed to tell a good story; frex, Return of the King could have cut 10 minutes from the end easily, and A New Hope -- which isn't an overly long movie -- could have lost ten minutes on Tatooine harmlessly), but I think anything much more than 2 hours ought to have an intermission, and that regardless of length, the actual movie should start within five minutes of the advertised start time.