The Word of Wisdom in Mormon doctrine states that all things should be taken in moderation. Fast food and other junk food leads to many deaths when people over indulge in it. If it was taken in small doses and healthy practices of exercise were incorporated in the partakers routine then it wouldn't have much of a negative effect on someones body at all.
Those who over indulge in fast food, sugar, caffeine, even over indulging in things like exercise or meat of any kind are against the Mormon Word of Wisdom.
Tea, like coffee or tobacco, contains substances that cannot be balanced in your body by other chemicals found within the human body or by exterior activities (like cholesterol can be with exercise, etc.). The full list of compounds can be found at:
http://www.innvista.com/health/nutrition/diet/tea.htm
There are of course some aspects of the beverage that can prove to be beneficial to your health, but this goes the same for alcohol.
I don't think we need to get into the negative effects that alcohol can have and why the Mormon church does not allow it.
The Word of Wisdom preaches moderation in all things and encourages a healthy life style. It promotes things that compliment each other in the human body as listed above.
Black and green tea, along with a few other disallowed substances in the Word of Wisdom, must be taken in very small moderation to have very small effect, but due to their specific compounds they have been put on the list of disallowed substances in the Mormon church. Even when taken in small doses over a long period of time can have a harmful effect on the human body.