I'm definitely not a "coke and chips" parent, but there were quite a few times that pediatricians told me not to give my kids juice for the same reason - and I nodded my head, thinking "yeah, right" while the pediatrician said it.
It wasn't until I started paying attention to my own calorie intake and trying to improve my own diet that I (an educated person) really understood why juice is something that really shouldn't be given to kids regularly.
I'm not sure exactly what a doctor could've said to me that would've helped me realize it earlier than I did.... although its possible that spending more time explaining why juice is bad (for me, this would be telling me what a kid SHOULD eat in a day, including average calorie intake, and then explaining that three cups of juice a day in addition to the correct amount of food will cause a child to be overweight in a relatively short period of time).
My current calorie counting habits have also led to us not eating fast food nearly as often as well.
Now one might say that just knowing whats healthy and whats not should be enough to make the right choice.... well, for many parents who don't think they're kids are overweight, they may not feel the need to make a change. That was my reason for not changing at the time, my kids were, (are) healthy, they're active, they're not chubby at all, why should I get rid of juice - I drink juice regularly (or I did) and I'm fine, whats the big deal?
Education CAN make a difference, but I can understand why it might be frustrating to the educator who might feel like their words go in one ear and out the other.