*of course we can. We can post a passage of the Old Testament, The 10 Comandments, while not posting part of the Quran which says Jesus didn't die.
What part of this Christian Nation's First Amendment don't you get?
You are correct.
If I allow posted the 10 commandments, then I should permit posted "Mohammed and the cow" or some such. (Bulletin boards, signs, etc.)
However, the decorator can stick up what the decorator likes. And the next decorator can stick up something different.