Leaving with a pot of yogurt

I was at a grocery store and had picked up a pot of yogurt. The cashier was on the phone and so I just nodded to her and walked out. And I thought wasn't that nice of them to let me out without paying? And I thought- can they do that? And I looked at the yogurt and wasn't pleased with it, so I went back in the store to get a different one and pay for it.