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If you picture the direction of load on the pedal spindle, it's fairly close to the plane of intersection of the thread shoulder and crank arm - for this face to be clamped tightly enough to the mating surface on the crank to preclude fretting, would require a totally unrealistic tension on the threaded section. This substandard design inevitably leads to damage on the cranks.
A chamfer would allow that coarse pedal thread to more adequately clamp the mating surfaces, which would then be more or less orthogonal to the load going through the junction.
Car wheelnuts have this chamfer for the same reasons - to increase the clamping force afforded by a given amount of torque through the thread, and to better orient the mating surface to the loads, greatly reducing the chance of relative movement.
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Kimmo
ffm
Why the need for a chamfer i.e. can someone explain this
"Jobst Brandt will be pleased to explain to you why the interface is so fundamentally flawed that no tightening torque can really be called adequate, since the pedals will loosen at any torque short of the failure stress of the thread."