Personally, I am a fan of the all-or-nothing approach.
You either allow all religions and their icons or symbols or you allow none.
Either we are a secular nation, or a pluralistic one. I know, it's not what most of us on FR want, but it is the reality of the world today.
I have no problem with devout Muslims putting something up (as long as its not a flaming model of the World Trade Center), or devout Jews putting something up. I don't accept their religions as valid but the Constitution and basic courtesy don't require that I do.
This business of barring only Christian symbols, OTOH, is blatantly discriminatory and must not be taken lying down.
But your point does raise the question: Is it really okay for the government to oppress every religion equally?
I tend to think not.