I don't think it's about faster waveform rates. It's more computing for the additional scaling (magnification) than a simple 1:1 mapping. I think it's a cheaper analog front end. The analog gain and offset doesn't need to be accurate set. The correction is done in software.
There is no such thing as a simple 1:1 mapping. There is decimation, interpolation, and the intensity-buffer. Any additional scaling of the ADC values will be done at the same time as it upscales for the display - just one scaling operation - that's why you can often see horizontal blank lines running through the waveform display when it does it. So I would think that having less values to interpolate, etc, would save time - but I don't know for certain.
But if it was just about the quality of the front end, why would the DS1000Z change the scaling when more channels are turned on?