Have you suggested this to Apple? I'm willing to do so too, to help bump it up a bit. Makes more sense for the iPad to do this, since it can be used in a 0, or 180 degree orientation. One of the small details that is interesting they missed, probably from the OS starting on a phone where 180 degree rotation wasn't supported.
180 degree rotation has been supported on the phone for as long as I can remember.
I for one, in many thousands of hours of usage, have never accidentally used the wrong volume button on an iPad or iPhone. I'm pretty sure I would have if the buttons changed function based on orientation.
Quite apart from anything else, with the screen off how would you know which orientation the screen was in !