I disagree. He might not be morally obligated, but his is certainly socially obligated. Customers ask for the wrong thing all the time. A problem solver, as any good retail sales person should be, will ask questions to help the customer understand their own needs and the available solutions. That story just shows that Dan Luu doesn't understand the social process that happens in the retail environment.

That he doesn't care if he looks stupid is besides the point here. He looked stupid to the retail worker not because he didn't explain his underlying reasons for wanting a small box. He looked stupid because he didn't understand the retail sales process. That is to say, he was stupid, about that one thing, and that is why he looked stupid.