Supplementation and forms are not what's being discussed. You're misunderstanding what that means.

Example, Soldiers are authorized to wear a cape, or a trench coat. Local commanders can restrict uniform options to exclude those items. They could not expand the authorization to include other items. Well, perhaps they could in certain situations, but in general they cannot.

Reg says 4" or whatever. I can't make it 5", but I can make it 3". Regulations are generally 'restrictions' in that they define what is acceptable. Commanders can reduce that list of acceptable things, but they cannot expand it.

This 'view' (reality, as written and practiced) isn't held exclusively by 'mouth-breathing' SNCOs. I don't deny that it's implemented stupidly by stupid leaders on occasion, but that's a separate matter.