It's OK to wear a cross, or a pentacle for that matter, in any public place you want, it's called freedom of speech, and it didn't cost the tax payers anything.

Placing a religious monument in a public building that's paid for by the tax payers of that state, especially without a public vote, is a totally different matter. I'd be curious to know who paid for the monument itself, as well as the cost to install/remove it.