For the same reason they're not accepting gift certificates... To limit the number of iPads purchased by an individual while supplies are still tight and limited to the US market. Did you not read the email? Steve Jobs is quite terse and easy to read.
I think we all read his email and understood it. The point is that this is not reasonable. The gift card was purchased in good faith that it could be used to purchase whatever products Apple sells, on demand. I have some gift cards that I'd like to sell you, but you can only use them when I say so.