I don't think that any of the low carb authors has said that drinking fruit juice is "unsafe". Only that they concentrate the fructose and allow you to consume the same amount of fructose found in about 4 oranges in one quick glass (minus most of the fiber, of course) and that for the amount of carbs you get in that one small glass of juice, there are far more nutritious choices that are far more beneficial to weight loss.
It's also sound advice to choose the whole fruit over the juice of that same fruit, but that would mean the sale of much less oranges, wouldn't it?