The First Amendment prohibits GOVERNMENT from making any law establishing religion or preventing the free exercise thereof.
These people can, in no way, shape, or form, violate the Constitution by expressing their written beliefs. You might make an argument that Whiteville was guilty of something if they had a permit letting them to do it, but due to the temporary nature of this display, you'd likely lose on that count too.
Janine Turner has a wonderful series of essays on her website, written my law professors from several well-known schools. You might want to check them out. You might truly learn something about the Constitution!
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