I'm not sure. My guess is that it is because the ambi-safety configuration is, or can be, manipulated (pushed on) from both the left and right sides. Consequently, if the installed lever on the right side is not properly supported as Fastbolt indicated in post #23, there is a possibility that the lever could move laterally to the left enough (and stay to the left) to interfere with the motion of the firing pin.

I would think the engineers at the factory would also be concerned about potential wear on the firing pin stem from the repeated contact with the safety body when the lever on the right side is regularly used. (left handed user) The ideal scenario is for the safety body to stay put in the center of the slide as much as possible. There has to be some gauge there for the safety assy to work properly, but not excessive movement side to side....hence the need for the tab on the right side ambi lever.

This is obviously not an issue with the single sided (non-ambi) safety because it is not (cannot be) manipulated from the right side.

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Ret. LE, FA Instr, S&W Armorer