I use the mobile website just fine on my iPad. It's not the same of course, but then it probably makes more sense to devote their time on the iPhone app more because that's where 99% of their customers are. It's complicated anyway because if you're doing purchases online, you're probably using Safari to begin with (even though there are numerous shopping apps) and you can't route that process to the app, especially now with all the new security stuff that adds an extra step just to open an app.
Why they don't just stretch it? I think that is actually against Apple's App Store policy. Plus, that just gives them that whole new pool of issues that the ~1,000 people using their app on their iPad will have.
That probably doesn't make a good excuse, but you know, it's probably saving them money to make it an inconvenience for us.