Sprite zero cupcakes

This recipe puzzles me. Sprite and Cola Zero are made with aspartame. It is specifically stated on the bottles that it is not for cooking or baking (and any experienced dieter would know not to use this for baking or cooking). If I were to use a diet soda, I would use RC diet drinks or any other that is based on Splenda. The other problem that puzzles me, is why use a ready to bake that contains sugar, and then use diet soda as a liquid. What is the purpose? Thank you for your reply.