You don’t understand why running the lap in the normal direction would not be a problem at 0 and 90 degrees?
Remember that the Fac-Ette has that large protractor for changing the angle. I think that makes it a lot different than many other types of faceting machines. Generally for use, the stone is at the 6 o’clock position on the lap rather than 3 or 9 as I have seen for other makes. The result is that the direction of the lap goes with the facet height and spindle axis and is pulling away from the spindle. When you go to zero degrees, if the direction was not reversed, it would try to pull the stone from zero. Running in reverse, no fighting the lap directions as the hard stop keeps the table facet where it should be.
For the girdle, it seems a lot easier to control the stone when the lap is pulling away. Plus any splash that escapes is headed away from the user.
If that still doesn’t make sense to you, perhaps you can explain how it might be otherwise. I have only worked with Fac-Ette, so I am not familiar with the behavior of other machines.