That's a misconception, if the fog lights were on at the same time as the HID headlights, you wouldn't "blind" oncoming traffic. The fog lights don't turn on when the HID headlights are on because the two lights "burn" at two different intensities, and therefore produce two different colors of light, which when shown at the same time, just looks ugly. This is the only reason they don't both come on at the same time.
In the future I'm sure someone will come up with a kit that lets you switch on the fogs while the HID headlights are on. However, if you do this, I would recommend also purchasing a kit to convert the fog lights to the same or similar light intensity as the HID headlights so that they look good together.