I have no idea how those dealers "checked" the tire, but it shouldn't surprise anyone that a Harley dealer tells you something is "normal". I'm pretty sure it's part of the franchise agreement and one of the first things a new dealer is taught, everything is "normal" until the mothership says otherwise. And the mothership only says otherwise after at least a year or more of trying to wiggle out of any responsibility.
Take your bare hand and cup it lightly over the tread, slide your hand over the tire in one direction, then back in the other direction. Does the tread feel smooth in both directions, or do you feel sharp edges when you slide your hand one way? The first indications, before the abnormal wear becomes visible, are found by feel.
Jerry