The town can probably get away with the religious symbols as long as they don't spend any money on purchasing them (they were probably purchased by the town long ago) and as long as they are willing to erect anyone and everyone else's religious symbols or non-religious symbols of the season. Perhaps someone will ask the town to erect satanic symbols and then they'll have to do so or face a lawsuit.
Satanic group says Oklahoma must give the devil his due
It is ironic that the town feels the need to promote a festival about a a mythical figure during a much older festival based on a scientific event.