Earlier quoted context omitted.
>I also notice that the power output of the USB ports on the monitors is barely enough to charge any recent mobile device. That's because it's probably supplying the USB spec of 500ma. That's not a fault, it's how it should work. The usb-looking chargers that come with many recent mobile devices are using a bastardized set of "not really standards" to let them pump more juice through the USB port than is allowed.
It's probably the cable, if they use a data cable it will limit it to 500ma, but if they get a usb power only cable I bet it will supply more. You're right its probably working as designed.
But some do allow it to go out of spec, and if that happens you'll only be able to get that extra power if you are using a "charging only" cable (which tricks the phone into thinking it's plugged into a power-only USB outlet and it tries to draw more than 500ma).
So with a USB cable (that's not actually a USB cable), plugged into a USB port (that's not actually a USB port) you can get over that, but it's not meant to run that way on several levels.
It's one of the things that USB-C is meant to solve. Let the device and power source negotiate on how much can be sent and received by each (even being able to swap which way power is going). Sadly that means that cables are no longer just a bunch of metal in a wrapper, they need a set resistance or in some cases built-in IC's, and that means that now a shitty cable can cause issues.