I think both of you are missing the reason for the separate USB connector for charging and why it’s a micro USB.
This is a portable device with an internal battery. In EU all mobile phones are required to have a micro USB charger. As this device will be used with a mobile phone it’s logical to use the same connector the mobile phone charger use. That way you don’t have to carry a different charger with you.
Similarly, if there has been a single USB port, the mobile phone would be charging the device, depleting it’s battery quickly. Furthermore, with a single USB port you wouldn’t be able to use the device while charging the its battery.
Get it?