really do not think it is Descriptor side fixed because the same drive that reports 2 mA here, will not power up on my USB enabled DVD player at home ( must use AC adapter ).
It is somehow, misteriously driven from the motherboard side, whatever.
It is the usb device that calls for power, your DVD knows it is to be powered from external power, it don't call for any power from the usb port.
Well the history of usb ports on older mother boards, was just that, 2 ports at 100mA each, so when you connect a usb device the device take upto 100mA and if it needs more it will call for up to 500mA, before needing to be powered from external supply.
On the newer mother boards, they added 2xhubs, so they will supply upto 500mA each port, to give a total of upto 4Amp (Load no problem for a desktop pc).
Thats why you get a lot of people give the wrong advise, to use a external hub, to solve driver problems.
There is a lot of examples to look at, from the way microchip use usb (see thier vast collection of Application Notes, also the info for the pickit2 has lots of info on the usb control, as to use usb power or from the device being programed)