It was not the presence of the creche that made it unconstitutional. It was how it was presented. In other words you can have a creche in the courthouse. You can't have it on the center staircase, with a prominent sign saying that it was donated by the Catholic Church, with an angel holding a sign saying, "Gloria in Exclesis Deo."

The presence can be justified as a secular symbol of a widely observed holiday. The manner that it was presented pretty clearly supports Christianity in general, and The Catholic Church in particular.

Sure, there is ambiguity in these dividing lines. But ambiguity is hardly new in our legal system. That is what precedent is for, and generally lawyers and judges who know the precedent have little trouble figuring out whether any given situation is going to be ruled OK or not. Without knowing precedent, you won't have a chance of figuring it out. Nor can anyone just show it to you written out in black and white, because it is the nature of the beast that any specific statute that you write will have grey areas that need clarification. Particularly once it has been read by people with different assumptions and understandings of language.

(Yes, a major problem for the legal system is that the English language changes over time. Words written in the 1700's do not always mean today what they did then...)

Cheers,
Ben