Why does my mainboard power the cathode to turn the Power-LED off?
This would be standard in all motherboards compliant with the Intel Front Panel I/O Design Guide.

Note that what you see as +5V is actually a resistor of a few hundred ohms to the +5V supply. 



The likely reasons include the practical (a short of either side to ground damages nothing) and possibly they were thinking a MOSFET drive could be used, so the higher carrier mobility of electrons over holes means an N-channel MOSFET would be preferred. I believe bipolar drivers are typically used at least on motherboards sold at retail- probably for ESD immunity reasons- no motherboard maker wants to have to take returns because an LED driver got fried by a sloppy assembler.