I don't know if it's an urban legend, but the story I had always heard was that chocolate became very scarce during the war, as milk was rationed, but Hershey's figured out how to make decent tasting chocolate from spoiled milk, which was easier to get since no one wanted it. The war went on long enough that Americans eventually expected chocolate to taste like that, so after it ended they reformulated to keep the signature taste while using fresh milk.