As a little insight into this from a technical side, here we go:
Companies that make devices with non-removable batteries have a challenge.  They need to protect the device from discharging too much but also provide a way to "reset" to allow it to start charging again.  Lenovo does this on their "sealed" laptops by providing a pinhole on the bottom to insert a pin or paperclip to press a button to reset the charge circuit.  Until that switch is pressed the computer won't turn on OR charge.  It sounds like Motorola implements a similar protection with the power/volume button thing, though I've always wondered if that pinhole at the bottom of the phone on the back is for this purpose also.