I think it is a matter of common sense really. If you had loads of smoothies just as drinks, then you would find that you were adding a lot of calories to your diet without really noticing. If you have one occasionally, and keep in mind that it is not just a drink but almost a meal, then it probably won't do too much damage.
For instance - a 250ml bottle of a certain well-known brand of smoothie will be between 120 and 160 calories, depending on the variety. And I don't know about you, but I can knock one of those little bottles back in an instant!
If you look at the ingredients for the strawberry and banana one, you have banana, strawberries, apple, peach, and orange - put that on a plate and you have quite a nice fruit salad, which would take you much longer to eat and would be more satisfying.
I do have smoothies from time to time, particularly when I am dashing around and haven't time to stop to eat properly. But I do see them as a quick convenient meal replacement, rather than as a drink.