It's not only weak, but by letting the team play with the bat "until it mattered" he jeopardized the safety of kids. And that makes it borderline criminal.
About 15-20 years ago they developed composite softball bats like the Miken Ultra. They balls flew off the bats and average sized guys like me started hitting 350 foot homeruns. It was fun. Until people started getting killed. Leagues banned the bats, and then players started to paint bats to conceal them. They also started end loading bats illegally by inserting weights in the end cap. I played in a game once again the New England Sports Plex - I was catching - and a guy hit a line drive right off the pitcher's face. They guy never even got his glove in an upward motion. Worst thing I've ever seen.
Illegal bats are illegal for a reason - because they aren't safe. The team deserved to be punished for using an illegal bat, but your coach was a horse's ### for letting it be used until something big happened.