Actually, the real explanation is not just with the parallax. With only parallax to take into account, all you'd have to do is add margins corresponding to the maximum parallax excursion, and be done (and turn off the feature if you don't want your backdrop to move).
The real problem is the fact that on the iPad the backdrop adjusts to the device orientation. There is just no (evident) way to change the orientation of a non-square,rectangular frame w.r.t. an image. Making the image square doesn't help except. You could scale the image so that the smallest dimension of the screen doesn't crop the background, but there is no automatic way to determine whether you have to scale down when going from landscape to portrait, or rather from portrait to landscape. That'll depend on the picture.
The only way to approach a workable solution would be to let the user set or tune the wallpaper for both the landscape and portrait orientations ... or an option to turn off the rotation.
I fear that the former option is giving too much customisation control to the user for Apple (they've never been about control over their design choices, with a couple of exceptions like Finder font size control to confirm the rule). That's something you accept when buying one of their products (and there are much more far-reaching implications of that than the mere fact you can't put that wallpaper of your granddaughter's first poopoo exactly the way you want it ).
As to an option to turn off the rotation feature ... someday maybe (though be prepared to have that fix all applications to one orientation in its first implementation!).
In short, this whole issue is a design bug ... and those are very easy to dismiss as "works as intended" (for the simple fact that that's the truth).
