"Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof."
I want to FREELY excercise my religion, and I want FREE electricity to do it.
So using the very same logic, if the government allows the occupiers free everything, in essence they are establishing the occupiers view point.
In essence making it the law of the land.
Now of course the argument would be that the government WOULD NOT be establishing it as law, just upholding their free excercise of speech.
Which brings us back to nativity scenes.
If in allowing free unobstructed use of government land and utilies is not an establishment of government on speech, but an upholding of freedom of excercise, then neither could it be an establishment of religion, but also a freedom of excercise.