I would agree that eat whole fruits and not overdue it on fruit juices, especially because they are 99% cooked/pasteurized and not raw, but do you think people are drinking that much fruit juice to gain weight from? An 8 oz serving of orange juice is only about 110 calories.
It's more likely people are drinking fruit juice with foods that don't digest well with fruit and/or those people still eating to much fat which are the highest form of calories.
8 oz carrot juice = 80 calories
8 oz orange juice = 110 calories
1 tbsp olive oil = 120 calories