It's a private museum. I don't see what the problem is. Even as a Christian, the separation of church and state is an important principle to adhere to, but this isn't a public building, and therefor this law shouldn't apply.
And let's be fair here. If the steel beam was just a regular ole steel beam, it wouldn't be a fraction as interesting. It's also a little harder to fraudulently duplicate. It's not likely that a building crash is going to create steel beams in the shape of anarchy symbols, stars of David, or ying-yangs, but if it did, that would be really interesting.