Yep, really surprised us, too! Never on any cruise had we witnessed something like that and like you mentioned, we've seen people denied entrance into the dining room not dressed appropriately.
And what really got me is that we were called the day before our reservation in the Emerald Steakhouse to remind us of our reservation and the lady told us that appropriate attire in the specialty restaurant was 'cruise elegant'. I asked her what it meant and she said nice cocktail attire would do nicely. Imagine our surprise when I showed up in a blue blazer, Oxford shirt, and Dockers, while my wife wore a nice cocktail dress and we were the most overdressed people in the restaurant! There was only one other gentleman wearing a jacket and most the guys were wearing golf shirts. The guy behind us was wearing a t-shirt, jeans, sneakers, and a baseball hat that he wore during the entire meal - not what I would call 'cruise elegant'.