Are you saying Janty Miswired the mini usb port and the cable they supply is not wired to the usb standard.
Let me put it this way...if Janty warns that using a different USB-A to USB-B-mini cable will violate your warranty, then that does indicate to me (from a technical perspective) that they are not following the USB standards.
Maybe they're pushing more voltage than specified.
Maybe less voltage.
Maybe they crossed a wire.
Maybe they used a higher or lower gauge wire.
Maybe the mini-USB port is wired a little differently.
There's hundreds of reasons and the "unknown" is on the Janty end. BUT, in the end, if you only modify the *supply* side effectively extending your port, then you are not modifying Janty's design and, therefore, cannot violate your warranty. People would be hard pressed to prove the extension cable you used was the root cause...you could also have a weak USB port on your PC.
Right now, I will say that I am using my Blackberry cable to charge my PCC and haven't noticed any ill effect. I think Janty is just being a little careful about what they say okay to.
They are concerned about people using different adapters and all kinds of things. In the end, just extend your USB port (supply) using the example cable I showed you and I don't think you will experience any problems.