It's more complicated than that. The iPod/iPad/iPhone connector is a 40-pin connector. In amongst those pins are pins for USB communication, and both digital and analog audio. And the audio lines include both volume controlled (by the iDevice) and line-out connections.
In order to sell a product that accesses all of the pins in useful ways requires that you pay royalties to Apple to get the proper communications protocols for them. They are not all standardized. Getting analog audio out isn't difficult, so there are lots of cheap products on the market with analog audio connections through dock connectors.
Personally, I use a cigarette-lighter based USB charger under the panel, and run a standard iPod charging cable out to the front of the panel. On the panel I have one half of a velcro patch, and on the back of my iPhone case I have the other half of the patch. iPhone sticks to panel, and gets power from plane. Bluetooth connects it to my Lightspeed. When I can connect it to my intercom with Bluetooth instead, i'll be even happier.