Guarana is a flavour that is allowed to be added to food ingredients. It's an approved ingredient as a flavour. It is actually not banned in the U.S. in these particular products.
In 2010, when the U.S. looked at these products, the companies voluntarily decided to pull all products that would be in energy drinks out of these products. That included guarana. It is not a question of banning. The companies chose to pull out all the ingredients that would have been in a traditional energy drink, one of which was guarana. Taurine is another. There are other ingredients.