No denying that. My question is why was this Marine supposedly singled out. My take, again, is that she was a problem child, and had deeper issues and was trying to hide behind religion.
That being said, even though this is the Marines, and what an NCO says to do, if lawful, should be followed, if said NCO allows other Marines to do what this one Marine is denied, then there is an issue. Especially something as mundane as workstation decorations.
Want to make it stick? Then ensure all workstations have no personal items on them. Problem solved.
When I was stationed at Osan Air Base, Korea, back in the late '80s/early '90s, we had several black Airmen who were wearing "Black to the Future" or "Black Power" t-shirts on base while off duty. Imagine the furor if a white person did the same. Base commander said all shirts of such type were banned on base, for all races/ethnicities. Problem went away, and no one could say "boo" about it.
So no, I don't think this issue was about religion, I do think it was about a problem child, but the Marines could have helped themselves significantly by ensuring no one had any personal items on their workstations. Period.
The issue in the 70’s was what decorations and personal items on display in rooms, especially posters. The easy out for commanders and First Sergeants was to simply ban all posters, but the Army was giving soldiers a little control over their little piece of the Green Machine. Better solution was for commander and First Sergeant to make decisions on case by case: if the poster needed to go down, down it came. In the end, the Army has the authority and can exercise it as required.