So they eat their ice cream and stay skinny. Big deal, for the rest of the day they might not be eating anything else, or else they're hitting the gym for 3 hours burning off the excess calories, thus remaining thin. You can't judge people based on what they eat for a single meal without knowing what else they do or eat for most of their lives. People don't go out to restaurants or bars or ice cream shops to eat broccoli and steamed vegetables and sugar-free fro-yo, it's a once in awhile indulgence for people who can restrain themselves otherwise.
You're overthinking it. One dish of ice cream is not going to kill you, nor will it make you gain 5 pounds from water retention or fat. The 20%'s there for a little flexibility and ice cream. Enjoy it.
F 28/5'4/100 lbs
"I'm not a psychopath, I'm a high-functioning sociopath; do your research."