David,
In response to your post, Cary is trying to protect itself legally. If it didn’t go after the pro-Cary message, it would indicate that their actions aren’t content-neutral, which would mean its actions would probably be unconstitutional.
Cary will need to argue that its sign regulations address time, place, and manner issues (content-neutral issues). If its restrictions on speech aren’t content-neutral, then courts are far less likely to find them constitutional.