Does anyone know what's the deal?
In some cases the chargers and devices have gone a third way. This is very typical for tablets as they need a lot more current in order to charge within a reasonable time frame. At that point many manufacturers have made up their own way of signalling the ability.
It worked with my brother's iPod classic (maybe 5th gen?), but not my nano; later I saw something on Adafruit about how the iPhone needed specific voltages on the data pins to charge, and I figured that was what was going on.
But recently, I got a battery pack, which can output to microUSB, iPod, or some other things. It works with my iPod, the same one I had years ago. But the cables it came with have swappable ends, I think 2.something mm jacks. At any rate they only have two points of connection. Here's what it looks like: http://i.imgur.com/8TliS.jpeg
So, how does this work? As far as I can tell, this should be exactly the same situation as a standard USB-iPod charger with its data cables cut, but it's not behaving the same.
It's possible I'm getting some details wrong, but can anyone shed some light on this?
You might have been caught by this same problem.
Also, the USB cable may be too high resistance, causing a voltage drop.