That's a common mistake. Most Mormon's aren't bothered by sodas, caffeinated or not. They are specifically opposed to tea and coffee which gets interpreted as caffeine. No, it doesn't make sense and a caffeine prohibition would at least seem to have some type of logic behind it (some of them do that)... but it's religion it's meant to make you at least a little stupid.
The rules are the first derivative of the intent. This haunts all religions. You will always get a special group of people who follow the rules without considering the intent, usually with the goal of feeling superior to everyone else.
Its called "legalism", and it leads to all kinds of nonsense.
