I agree with you, I think this was very rude. It's one thing to eat just the licorice handle off your *own* cupcake. It's another to take a very small portion of every piece of food remaining. It's like somebody had made those peanut butter cookies with Hershey's kisses in the middle, and somebody went through and ate all the chocolate but left the cookies. Or if somebody scooped all the icing off all the remaining cupcakes and just ate that. Rude, rude, rude.
Now, if he'd been allocated one cupcake, and all he ate was the licorice handle? Fine. A bit wasteful, but his own food, etc. I do think it would have been inappropriate for him to take another cupcake, though, if all he ate was the licorice. I think it would be impolite to take second helpings, or more, if you haven't eating the majority of your first item. It's a fine line to draw... would eating the cupcake all *except* the licorice handle be okay to get another one? I'd probably say so. I guess I'd expect a person to eat "all but" something, like at least 80% of the item, in order to get another helping. And even then, I'd keep it to two helpings.