It is not proprietary. It is a standard mini-USB socket. Standard USB Ports are not used on devices like mp3 players, printers, etc. The common USB Plug on wires which we plug in, are USB Series A. Both the ends of the wires are not USB, because A/A USB cables with no bridge between them can burn the USB controllers on the computers and even lead to SMPS failure. So, they are avoided and the mini USB is made use of. And they are cheaper too.
Charger is a charger, it doesn't mean much if its original or duplicate. You can get an original from Motorola too, they would be pleased.