Apple has been a bit overprotective of their hardware and does not want anyone taking advantage of their physical buttons. Pulling a nifty little trick like that can get an app nixed from the app store. This certainly was the case for two iPhone apps, Camera+ and Quick Snap
Camera+ a top app in the app store got the axe because they had hidden, an apparently desirable feature, where the volume button on the iPhone acts as a camera's shutter. Camera+'s "VolumeSnap" feature was enabled through a work around by entering the url "camplus://enablevolumesnap" into the Mobile browser. Apple rejected the feature because it used the volume button in a "non-standard way, potentially resulting in user confusion" Camera+ reported generating over $500k in its first two months. Luckily for Camera+, Apple was forgiving and allowed them back in to their home, the app store, after defying their rules and experienced 78k downloads on Christmas Day.
Following in Camera+'s footsteps, Quick Snap attempted this same feature but with some hubris. There was no effort to "hide" this feature, in fact they show it off in a screen shot. Quick Snap took it one step further and makes use of the "-" volume button to trigger a time shot. Quick Snap got taken down in just about half of a month. Some people just never learn. Maybe next time Apple say's no button remapping, people will listen.