Whether you like it or not it is a great marketing strategy. If you have a HOT selling product and make it a little hard to get people want it even more. The last thing you want to do is just flood the shelf with iPads. I think they learned from their first iPad launch when plenty were left over after launch day to make it a little harder to get, thus driving demand. Things like that make something "gotta have" in the eyes of the general buying public.
stupid theory
at some point the initial demand will go away and they don't want to pay for increased manufacturing capacity that will not be needed in 6 months. they have to guess the weekly/monthly demand a few months from now and have Foxconn manufacture to those estimates.