You are right in that a room where a judicial proceeding is taking place is off-limits by law (39-17-1306). You are also correct that it doesn't mean the whole courthouse is off-limits.
However the posting law (39-17-1359), allows any local, state or federal government entity to post their property in addition to privately owned property. So the county can legally post the entire court house.
I would contact your county mayor about the sign and/or go to the next county commissioners meeting.