Awful behaviour, but the issue of lounge etiquette is indeed a tricky one. Is it acceptable to take anything out at all? I hardly ever do- though once I did sneak a mini Diet Coke- I had taken it to drink in the lounge but then saw that my flight was boarding and put in my bag. I felt a little naughty, but I had intended to drink it there and then, but then ended up leaving the lounge rather sooner than I had anticipated. Was I wrong? What if I had taken 2? Where do you draw the line? The case described by the OP is an extreme and pretty obvious case, and unacceptable. But the parameters are not always clear.
I am also one of these people that frowns on loading up a plate from a buffet to the point that food is falling off it. I prefer to put modest amounts on the plate, but to make several trips. But I know plenty of people that see no logic in this and prefer to be more efficient by piling food on plates and saving themselves extra journeys to the buffet.