Dishwasher soap is an abrasive, which is why some coffee cups with cheap logos on them wear off, and aluminum pans can get dull. I would assume there is concern with damage to the stainless steel, and or the seals on the cups. I was them in the dishwasher, my wife and kids have too many of them so if one gets messed up and I have to throw it away it is a win….lol
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