The best explanation I've seen came off another forum 5 or 6 years ago.
"Guys, this is a Law Enforcement contract overrun. Federal HST, Winchester Ranger Bonded, and now Speer Gold Dot have all found their way to retail channels in this manner. It says not for LE use because it's it's supposed to be for the contract order and since the contract buyer is not taking them it is only to be used in retail channels and not used in any other contract or by any other agency. Each contract has a specific requirement they ask for in the order. The manufacturer would like to not risk an LEO using this round and having it be outside the department requirements and the blame going to the manufacturer for such a round being outside the department requirements for a duty load. There is absolutely nothing wrong with it. The "Not for LE use" sticker is a save your ass liability sticker, so that if an LEO uses the round in a shoot, and after investigation it is deemed the ammo falls outside the very specific department requirements for velocity, they won't be liable for any damages because it was marked to not be used in that capacity.
"