Pyrotechnicians commonly work with far, far more dangerous compositions, flash, whistle, etc...they do explode in open air and are friction, static, and impact sensitive, yet they can be worked with safely and never have accidents if they use common sense and common safety.
I have worked with such things myself.
Hand grinding a highly combustable mix of dry chemicals just does not pass the safety test, even a mixture only marginaly susceptible to friction ignition.
Working with a pile of it by hand is different than intentionaly applying large amounts of friction to something which is intentionaly unstable and can explode or in this case deflagrate, while standing over it, with your hands and arms right in the direction the heat and energy would be directed. A minor impurity in the mix could leave the person disfigured and possibly disabled for life.
Making black powder can be safe. However an error, even not due to the individual and instead the supplier of a chemical or an impurity from another item getting in the mix can be disasterous.
In a mill the low chance of problems from someone taking proper precations is acceptable in small batches, especialy in a mill located in a location that even a minor explosion poses little risk of harm to anyone.
In a mortar and pestle that low chance just became unacceptable in my opinion.
Even factories that produced black powder were made so that in case of an explosion the forces would be directed in a harmless direction, usualy through one wall intentionaly built to give.
While this would instead burn rapidly and not explode, the same concept applies. The direction of the burning and heat would be straight up from the pestle. Right where the hands, arms, and possible face of the person is.
That is simply not a safe practice.
If we have different opinions on acceptable risk that is fine.