a friend of mine was telling me just the other day about a friend who offered his services to the government for free if they ever needed to make a sign. He spoke Thai very well too. They never took him up on it and continue to make nonsensical signs all over town. I think he was just trying to score some govt. connections but you might be right. They probably think it would be lowering themselves to a certain level if they admitted that a native speaker would be better at this than they are. Who knows?