Some people are "lucky" and can drink this stuff without problems. The drink itself may not knock you out of ketosis, but apparently the flavourings can cause "cravings", and these will play havoc with your mind if they happen, and it is the result of giving into them that will knock you out of ketosis. This, I believe, is why CD say no to these drinks.
If you think you are one of the "lucky" ones then risk it if you feel you need something like that, but just be aware that if you start to suffer more from cravings then get the hell away from the stuff as fast as you can.
I used to be a fizzy addict, but 95% of the time I steer clear now and make my own with SF juice and sparkling water and such like. Note, I am not SS, SS+ or anything like - just calorie counting at the moment!
Steve