This is because most users need this USB port to charge devices like smart phones and tablets; if the port had Z-Wave control, there's always the chance that it will be accidentally turned off in your app.
The charging/charged status in your Z-Wave hub's app will show as on and off. "On" means the connected phone or tablet is charging, and "off" means it is charged.
When charging a device with the USB port, it's best to keep the device off. Otherwise, it may start drawing power shortly after it's charged and send confusing reports to your hub.
The maximum load for the USB port is 2.1A, so it's not recommended to exceed this capacity.
Let us know if you have any questions about your Double Plug!