Most of Apple’s default iPhone apps don’t rotate when you hold the phone upside down. Many will rotate to landscape, but some don’t even go that far, such as App Store and Weather.
However, there is at least one situation where holding your iPhone upside down makes sense: Voice Memos. Since the main microphone is on the bottom of the device, this app deicides to allow this seemingly taboo orientation and kindly rotates the interface for you.
Music seems to be another case where Apple saw fit to allow an upside down orientation, but only if your device is plugged into, and playing music through, something like a car stereo. Maybe Apple accommodated for that kind of situation where the device often needs to be placed in a strange orientation in order to fit into a car setup.