This shouldn't be too surprising, as caffeine itself has a flavor. It is somewhat bitter, which is one reason why it's particularly common in root beer.
Did you mean that it's not common in root beer? Wiki says that root beer has little to no caffeine.
Root beer is so sweet that I would think it could mask the bitterness. I wonder if the plants that originally were brewed into it just didn't have any caffeine.
Do the ingredients to coke naturally have caffeine, or was that a substitute for the original alkaloid? I.e., is decaf coke because the removed the caffeine, or is caffeinated coke that way because it was added?
Coca-Cola lists caffeine as an ingredient in the UK.
I don't know why caffeine was originally added - maybe for flavour, maybe for the stimulant effects, possibly because of having to not have cocaine in it anymore...
