I'm on weightwatchers and so I definitely look around to see what other people are eating. But I don't fill my plate except with salad and a few choice bites (not even servings) of things I want to taste.
At the end of the day, it is their life and their choice. But I don't think food should be wasted regardless unless the person doesn't like it. We also have to remember that people are on vacation and do let themselves do things they might not otherwise do. So for this, I give most people a pass, assuming that they are just indulging themselves and do not always behave in this way. However, being rude to others behind you in line, by taking the last 10 pancakes, is not acceptable.