I honestly don’t remember the rationale but there were physical traits that are (were?) required for service. And being completely toothless was an easy disqualification, short of breaking or removing a limb, etc.

I recall the tearful horror stories and how this was done without any novocaine — to explain the level of sheer desperation and fear that led them to do this. Because they knew they’d die if they were forced to fight, and end up leaving my mom and her brothers orphaned