JOEL - Our latter-day church leaders have interpreted the 89th section of the D&C to include black tea and coffee . Whether it is the tannic acid or the caffeine or the fact that it is hot, doesn't make any difference. The point is, God has commanded it through His servants and therefore we should follow the principle out of obedience and out of respect for our bodies. We have been told our bodies are temples(1 Cor. 6:19) and that no unclean thing can enter God's kingdom(3 Nephi 27:19). That is the main reason. The advantages regarding our health are important but are secondary to this.
Our Church leaders have never specifically included cola drinks or chocolate in their interpretation of the scripture. However they did provide some council regarding cola drinks in a letter from the First Presidency in 1972:

"the Church has never officially taken any attitude on this matter, but the leaders of the Church have advised, and do now specifically advise, against the use of any drink containing harmful habit-forming drugs under circumstances that would result in the acquiring of the habit. Any beverage that contains ingredients that would be harmful to the body should be avoided."

The concentration of caffeine is lower in chocolate and most cola drinks, and that is probably why they were not specifically included in the main interpretation of the revelation. The main message we should take out of this is to not let our bodies become addicted to any harmful or habit forming drink or drug or food.
If you want some medical reasons for avoiding tannic acid and caffeine, I will offer a few.
Our digestive tracts are conditioned to work with the acids normally found in our bodies. However tannic acid is not one of them and can therefore be an irritant and lead to incomplete digestion. Tannic acid also reacts chemically with vitamin B1 and can therefore reduce the amount of available B1 which our bodies need. Large amounts of tannic acid can cause liver and kidney dammage.
People with hypertension, heart diseases, and women who are breast-feeding, should avoid caffeine, because it raises the blood pressure in the forebrain, accelerates the heartbeat, and reduces the milk secretion functions.
We want to keep our bodies as healthy as possible because we know from experience that the strength of our spirits can be affected by the health of our minds and bodies.
Perhaps the reason why you are confused is because you want someone to specifically list everything we can or can not eat. I don't think God will ever do that. In fact He has told us:

"For behold, it is not meet that I should command in all things; for he that is compelled in all things, the same is a slothful and not a wise servant; wherefore he receiveth no reward." (D&C 58:26)

So in accordance with this advise and to make it easier on me, I personally have decided to aviod all drinks, cold or hot, that have caffeine in them. Then I don't have to worry about it anymore. But I figure something that tastes as good as chocolate does, is probably going to be in heaven anyway, so I have no problems with eating that. But that is just my opinion.
Having decided to live the principle in the way I have stated above has eliminated any confusion I might have on the subject. I can now concentrate on more important things. And it is not my place to judge how anyone else decides to live the principle.