Why do most diets seem to want you to avoid caffeine? Is it fattening?

I’m just curious as to why most of the diets that I have been on in my life have wanted me to avoid caffeine. When I was on the Atkin’s diet some years back it advocated avoiding caffeine. Now that I am trying out the "raw foods" diet, the guidlines to it advocate avoiding caffeine as well. Why is this? Is caffeine fattening? I can find no information to tell me that it is for a fact fattening available online, so I’m turning to you people for an answer. Thanks for the info!