Based on the tests you've done, I'd recommend the following:
1. Just to be 100% sure, restart the system into BIOS by pressing <F2> during the boot process. Once in the BIOS setup utility, press <F9> to restore BIOS default settings. Reset any customized BIOS settings. Clear all DMI event logs by setting the option to "Enable", which is located in the Advanced/Event Log Configuration section of the BIOS Setup utility. Press <F10> to save the new settings and reboot the system. Test.
2. If the problem continues, test on any of the USB ports on the back panel of the board.
I've finally got around to testing this. Reseting the bios settings like described above doesn't do the trick. All usb ports don't get power when in hibernation mode.
Hibernate means save current operation system status into hard disk then shut down.
It is different from Sleep, only PS/2 keyboard or mouse can power on a shut down PC.