Originally Posted by
choddo
Why would the 45° interface mean the left pedal could be right-threaded. Does it change the nature of the rotational force on the pedal where it meets the crank?
The ~ 45 degree interface would stop fretting and increase friction - thus enabling the left pedal to remain in place using a standard thread (and allow for making flat and symmetrical SPD pedals same for the left and the right ones).
The similar design is used for automobile wheel bolts (or nuts - depending on the wheel mount design, but there's always that tapered interface).
Relja