Because the biopace makes it easy in one part and hard in the other. If you dont preserve direction, you may make it hard in the part of the pedal stroke that is already hard.

But on a fixed gear, your feet should always be tightly strapped to the pedals. I love biopace for platform pedals, on bikes where you need a bit of help to get through the weak spots in the stroke. On a fixed, it is rather silly.