... why the USB ports remain "hot" when the computer is shut down...
... As long as the power supply on the computer is switched ON, the USB ports are hot. At least, the one USB port on the computer that I am plugged into is hot...
... This behavior is the same whether the USB hubs' wall wart is plugged into the UPS or not.
It may be that the only thing actually powered up in the USB system is the LEDs on the hubs. My Kindle and my iPod both show no connection (either charging or data) when the computer is shut down...
Some modern computers when turned 'off' specifically maintain power on some or all USB ports to allow using those ports to charge USB stuff (cell phones, etc). It also enables USB keyboards with a
Power button to actually turn on the computer.
In some cases there is a mainboard jumper or BIOS setting to enable/disable this persistent USB power feature.
Many USB hubs will power themselves from the upstream USB port (your new computer) if they are denied direct wall wart power. The amount of power available to devices
downstream from the hub is greatly limited, of course. Hence no charging of tablets.
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