Fruit juice IS sugar, whether they add HFCS or not. There's really not significant difference between kinds of sugars biologically, the amount of sugar you can take in from juiced fruits is astronomical.
Just for completeness: The actual fruits are fine, though. Something (fibers play a part, but apparently can't explain the whole effect) in the whole fruits changes the absorbtion.
It takes 3-6 apples to make a glass of apple juice. It's easy to drink a glass of apple juice and want another but nobody is going to eat 3-6 apples. That's a key difference.
Fruits are artificially evolved to ever higher sugar content. They are better than candy, but they are also dangerous in excess.