JOEL - A lot of things related to our health code, called the "Word of Wisdom"(D&C 89), are left to the judgment of the individual. In order to remain in good standing with the church and be allowed access to our temples, we have been specifically told that we should avoid alcohol, tobacco, illegal drugs, coffee, and black tea; the harmful ingredient in coffee and tea being the caffeine(among a few other things). Our prophets have told us that this is what God expects from us and we follow it out of obedience to Him.
On the other hand we have not been specifically told to avoid chocolate or caffienated soft drinks which also contain small amounts of caffeine. In fact a 1.5 ounce Hershey's milk chocolate bar contains twice as much caffeine(10 mg) as one 8 ounce glass of decaffeinated iced tea(< 5 mg). So if a Mormon tells you he can't eat chocolate or drink decaffeinated tea he is going on his own opinion of what he feels is correct adherence to the principle. Others are not as strict on themselves. We have been advised by our leaders to not drink anything containing harmful drugs, in a way that would result in a habitual use of the drug (1972 letter from First Presidency). Church members follow this advice according to their own conscience and personal level of obedience. The overall principle to keep in mind is to remember that our bodies are the temples for our spirits and that no unclean thing should enter.(1 Cor 3:16-17)
Another secondary issue to consider is that we have been encouraged to avoid the very appearance of evil (1 Thess. 5:22). People who see a Mormon drinking any kind of coffee or tea are not going to know it is decaffeinated and might perceive them as hypocrites for not being true to their religious principles.