Being poor doesn't somehow cause you to drink soda. Water is still much cheaper.
Mexico is the second fattest country in the world (http://en.wikipedia.org/wiki/Obesity_in_Mexico), and soft drinks are used heavily there.
There are no calories in water. When your brain is inducing your body to increase blood glucose, you won't crave water. Sugary drinks are some of the most calorically dense (wrt volume and cost) foodstuffs you'll find. Poor people don't drink sodas because they are thirsty, they drink them because they are hungry.
A lot of very poor countries in Africa and South America have high obesity rates. They can only afford cheap starch and not enough meat and vegetables.