I agree with the opinions expressed here, but I've always wondered when this would come up.
A person would expect coffee or colas to contain caffeine, and that's why there is a huge market in decaffeinated coffee and cola products. A person also knows how one, or five, cups of coffee might affect them.
I'm not sure that the average teen, or older adult for that matter, is closely reading the label of flashy soda-like cans at the local 711. Especially if they taste like soda or fruit juice. Additionally, there is not a lot of regulation that ensures this stuff ( not Monster brand specifically) meets, or does not grossly exceed, label claim in the first place. Or that the ingredients themselves meet certain quality standards. Some of the products at convenience stores, and at supplement stores, scare the hell out of me. I've worked in a very highly regulated segment of the nutritional business for many years. We are an exception though...although that is changing.