Modifying the design of the bat, while passing if off as an original, is a violation of the manufacturer’s patent and copyright. All bats approved for play in the ASA, USSSA, NSF, ISF, NCAA, Little League or any other governing body bear a certification mark which means that a particular bat model has tested in a rigorous laboratory experiment in which its performance has been measured and found to fall below some set standard.
These certification marks are federally registered trademarks, and altering a bat so that it exceeds the certified level of performance is considered a federal crime.
Prep Blog: National federation toughens baseball bat alteration language
For it to be criminal one would have to profit from using someone elses trademark, it would be a fraud case. An injury caused by such a bat would be subject to civil action, in other words you will have to sue someone to get any redress, but no police agency is going to come to a ball park and arrest someone for using a bat that is not within the rules of the game.