I could have sworn I read on folklore.org that it was for speed, but I'm not finding it. You do have a point. Maybe Atkinson thought his dithering was more elegant without the multiplies or preferred how it looked on his test images.

I only used a B&W Mac a few times, but I do remember Windows 3.1 doing on-the-fly ordered dithering when running with palettized color (and being very surprised at NOT seeing the dithering on the blue gradient of a setup program once I started using high color modes). Windows 1.0 apparently was capable of doing it as well.