I hadn't seen that the people who wanted to post something on that sign owned it. I also haven't seen that the posters of the message own the land. If they do own both the sign and the land it is on (as opposed to leasing the land), then I don't see how anyone has any legal standing to challenge the sign that they want to post.
If there is a lease involved, then the issue becomes what the terms of the lease are.
free dixie,sw