I ask this question then: if there had been a warning on the bat, what would the kid have been done differently? What would the batter have done differently? Answer to both: Nothing.
The answer: Bought a wooden bat?
Metal vs. wood probably wouldn't have made a difference.
What would you have done if you were in the jury? Would you have said that "metal vs. wood probably wouldn't have made a difference", or would you have looked at whatever evidence is presented to you? I somehow can't imagine that they had a dozen expert witnesses who said "metal vs. wood probably wouldn't have made a difference" and then the jury said "Ok, it didn't make a difference but this is a big corporation and the poor kid is dead so we say they are guilty". I would imagine that the jury was shown believable evidence that there was a difference, and therefore the company was at fault.
1. Reaction time: If you shoot a gun at me, the speed of the bullet doesn't make any difference. But in a baseball come, if someone swings the ball right at you, reaction time is important. I would think there is a certain amount of time where you can evade very easily, a little bit less and you will get hit sometimes, and a little bit less again and you have no chance to escape. And I would think that a traditional pitch is laid out so that usually players had just about enough time to avoid being hit (because the game is no fun if you're hit all the time). And I could imagine that with just slightly faster balls it suddenly becomes very difficult to evade a ball.
2. Speed vs. damage: I could imagine that if your head is hit by a ball, there could be either very little damage or a lot of damage, depending on speed. That there is a speed X where anything below X is harmless although painful, anything above X is lethal, and very little in between. Again, quite possible that aluminium instead of wood just puts the ball above the threshold from harmless to very dangerous. I don't know about skulls, but I know that if you hit a cocoa nut with the flat side of a hammer, there is either no damage at all, or the cocoa nut splits wide open. Very hard to damage it just a little bit. So a little bit of difference in speed could be a major difference in the outcome.