Why would High altitudes (low pressure) require MORE voltage???

Rick,

 

Yes and, as Dan pointed out, regular mags at high altitude have the capability of internally arcing - increased voltage would only exacerbate that problem.  Also, no matter what the compression ratio is, the compressed combustion ready mixture is of a lesser pressure at high altitude than it would be at sea level. A measure of that change is MAP.  I suppose that both devices, plugs and mags, are subject to easier arcing as the pressure environment decreases - as long as the desired arcing continues to occur at the plug tips instead of internal to the mag.