Why bother?

In Costco the other day. At lunchtime. Was in line to get some unhealthy food. Hey it was quick.\!
There was this ~larger~ lady in front of me. she ordered 3 hot dogs!! 3!!! /?These are almost foot-longs!! Guess what she was drinking? Diet coke! It was only her, cos u get a drink with every hot dog.. she gave the other cups back.
Now, maybe I'm just misled, but why do people who are way overweight "prefer" the taste of diet drinks?