Why do no external iPad/iPhone batteries charge via a dock connector? There are many external battery packs which have a USB port for charging your iPad or iPhone on the go. But they all seem to be charged via a Micro USB port themselves, rather than a dock connector.

Why?

Is there some technical reason or does Apple not license the female dock connector?
Actually, many do.  Here's one example, and here's another.

The reason is probably licensing.  Apple charges a fee for developing products using this connector, and I believe there's also a very small per-port royalty (as with FireWire).