I didn't say the riser is pulling 12V from the motherboard, that's not the point. As I showed, it's the fact that the riser IS sharing an electrical connection with the motherboard and all electrical connections to the motherboard must come from the same power source to avoid conflicts in regulating the voltages from having different power sources. PSU's also supply 3.3V to the motherboard directly. There is no need for the motherboard to create it.