In moderate quantities, you are probably right that it doesn't do harm. On the large scale (that many people consume caffeine i.e. multiple cups of coffee, energy drinks, etc.) causes lots of problems, from addiction, to heart issues, to being linked to cancer. Just like any addictive substance, there is wisdom in avoiding it.

I tend to agree with you that obesity is on par with the harmful effects of caffeine. In that regard, yes it is a little ridiculous to see people eschew one and not the other. At the same time, why fault someone for avoiding something they know can be harmful, even though they have other problems. Should we all just embrace all sin because none of use are perfect? (no, I am not calling caffeine sin, it is a metaphor)