Because it wasn't lawfully posted outside the entrance nor inside the vestibule in a conspicuous/obvious place.
As to the next post, I reported it as a courtesy. You may beat the charge, but you can't beat the ride. Odds are very high that if anyone reported someone for carrying in there they'd end up cuffed, stuffed, and spend several hours at a minimum as a guest of the Lubbock PD.
Fortunately I have a couple of friends on the force in senior positions who could have interceded on my behalf and explained why the sign was illegal but the average guy wouldn't have.