Why do I get "no volume available" when using an iPhone 4s in a dock?
Yet the the dock is designed to just provide charging power and to hold the device, there is no line out on the dock.


This is incorrect. The description for this device on Amazon (sorry, I couldn't find any information on it that wasn't on a for-sale site) has this feature listed:


Built-in, 0.125-inch stereo line-out audio jack lets you attach an audio cable directly to your stereo, or run the included cassette adapter into your stereo's tape cassette player


Apparently the device does have an line out on it some place. And this is why your headphone and volume controls are disabled when you dock your phone in it.