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Because when you apply the brake, the arms move forward / towards the frame, i.e once the pads make contact with rim, they are pulled in a forwards motion, in theory increasing power.
On the front, they'll end up wanting to move in wrong direction (because the pads are "pulled away")
Keep em' on the rear ! I had these on my Ridgeback back in the day, & have a NOS set in a box somewhere. (as a set, the front is std, not "self energising")
There used to be a front version ( not made by suntour but Scott/Perdson), this had the 'right way round' helix but came with the problem that if you had a small buckle in your wheel then the brake would catch onto rim and then be drawn in rapidly by the helix and would throw you over the bars, bit of a design flaw! Suntour didnt want that liability to beresting on their shoulders.