In addition to avoiding interaction, I also prefer not standing there awkwardly while people work for me. I can't help the checkout clerk scan my groceries faster, and (if there is one in that lane) I'd just get in the way of the baggers, so I just have to stand there letting them serve me. I don't like that feeling of power/superiority/(not quite sure how to phrase it). It's the same reason I don't like mani pedis.