This is got nothing to do with religious practices. It is all about uniformity and sealing the protective mask. Doesn’t work with a beard.
Yup! Hell, there should be at least one Sikh in every platoon. Early warning to go to MOPP 4.
The mask sealing bit is pretty much hooey, because a large number of personnel have faces that are too narrow for an effective seal. They supposedly had extra small masks for them, but they never worked.
The Army has spent tens of millions of dollars in developing protective masks, and the standard mask is the best they have ever had, but it still is far from satisfactory.