I've been a first responder, you don't get 30 minutes to an hour to cut and dry shave in the middle of a hazmat response.
IIRC, troops deployed with MOP gear in both gulf wars, and had to drill in it too. Having this policy then would have involved a lot of bitching and potential reassignments. What happens when your critical comm specialist needs to be reassigned because he doesn't want to shave?
There's a LOT of crap our warriors prepare for and subsequently don't encounter. The purpose is so when you DO encounter it for the first time, it isn't a battle loser.
That was a bit “Fly off the handle” there, particularly in light of some of your other posts. Sorry for that...
Other Armies allow beards and train with gas masks. I am guessing that it works OK or they would not allow it. FWIW, I just tried mine on and it seems to seal OK with beard.