Those I know ... because they are tired of the grey, dreary, overcast skies and want to see some bright, clear skies instead. So Florida or Hawaii. Generally depending on if they have kids (Orlando, FL) or not (Hawaii).
Because Americans are conditioned from school to think of summer as when we take vacations. Especially if you have kids in school.
My parents once took us out of school a month early, for a family vacation driving across the country. But my brother and I still had to turn in homework, daily, by mailing it back to the teacher.
Yes, you are right about Americans and summer vacations but, I have broken myself of this misconception. I now vacation predominantly in early autumn, usually October, as, it's post summer rush and pre Thanksgiving/Christmas rush and the kiddies are all in school. Such an ideal time to travel!
Lemmings. I'd go to Queensland in winter if I could afford it every year. It actually gets hotter in Adelaide in summer, but it's a dry heat. Humidity is what kills you. Tropical summers are like stepping into a sauna.
Walt O'Reagun 