My husband and I went to [Insert Name of Popular Seaside Resort Restaurant Here] for breakfast. Several groups of diners brought in coffee in paper cups. Isn't it rude to bring food from one establishment into another? I saw it a couple other times this summer. When did this start? I felt bad for the wait staff who had to clean up the paper cups. If the customer left them, I might say "Excuse, you left this cup. You brought it in. So I assume you want to take it home?"
Etiquetteer does understand how important that first cup of coffee is every morning, especially at Popular Seaside Resorts. Nevertheless, it's never been Perfectly Proper to bring your own refreshments into a public establishment*. After all, restaurants are in business to make money by selling food and drink. Why on earth should they tolerate Interloping Caffeine?! Coffee Cup Capers like this need to stop at the restaurant entrance. Toss that cup in the trash, or at least refrain from going inside until you've savored every last drop.
Etiquetteer must restrain you from engaging in Disingenuous Banter with these scofflaws. It's already clear that they are Beyond Embarrassment, and you don't want to Cause a Scene. But it is tempting to contemplate lobbing that cup at them with a "Oh here, you forgot this!" That is why Rose Sayre said, in The African Queen, "Nature, Mr. Allnut, is what we are put on this Earth to rise above."
*It's truly rude to do this in a private home unless the invitation has specified "pot luck" or "BYO."** It casts aspersions on the hospitality being offered. Etiquetteer's coffee-brewing prowess has been insulted this way more than once - but then Etiquetteer also recognizes that one of the beauties of Friendship is Mutual Forgiveness. And Etiquetteer's friends have so much for which to forgive That Mr. Dimmick Who Thinks He Knows So Much. . .
**Vegetarians follow the example of the late Gloria Swanson, who would slip her vegetarian sandwich (or whatever it was) to the butler on arrival, whispering "Please serve me this when everyone gets the entrée." Observe that she didn't go announcing it to All and Sundry, including the hostess, but handled it discreetly. You'll observe how helpful it is to have staff on occasions like this.