In the OP's defense, Look and Shimano used to have a partnership in which Look built the pedals for Shimano. The Shimano 105 pedals I used to have used Look cleats.

Would it have been any better if he was looking for brake pads and told the clerk "I have Shimano brakes, so give me the Shimano brake pads (instead of the Kool Stops she handed him), you idiot."

I'm just saying that some things actually are interchangeable. Maybe he should have known that his cleats weren't interchangeable, but it doesn't mean he's a jerk for trusting that the clerk might know something he didn't.