I can only really speak for the high school side of it.
The HS coaches have a reputation of the school, and themselves on the line. I have had a high school coach take home and inspect my DD's bat to make sure I was not doing anything to it.
Sure, a player may have a "hot" bat that looks legal, and probably is. The coaches will continue to let her play with it.
But in high school, I have never seen a whole team using a hot bat that sounds wrong. That is just plain stupid.
When you are considered a top shelf area HS team, the others are watching you very closely. Many are very knowledgeable. You will get caught if you are this dumb at the higher levels.
12U travel, not so much. And possibly in lower level HS play to try to get an edge. I repeat, at the highest level HS game, too many eyes out there. Beat the opposition fair and square tends to be the only/best way.
The fans/umpires are too smart by then. I have seen just above everything, and know what to look for. I am not in the minority either.
Umpires are for the most part, not smart on these issues. They don't know enough about equipment, and many fans think they know too much...haha. We will agree to disagree on this post.
Also, I have been around as much as anybody and involved in every aspect of bats. Played for 20+ years and still do at a high level. I am a Rep for 4 manufacturers, I own a store, and have dabbled in the FP coaching by teaching hitting individually.