They say it interferes with gear and brake cables on S bars, AIUI. I have no gear cables (at the moment, this is due to change) and I'm running non-standard brake levers (Paul Components E-Levers) which may alter things.
I have a C bag with S type handlebars and it's never really been an issue. Some people suggest chopping the plastic handle off but I've not found it an issue.
I still find it disturbing when I turn the handlebars and the bag stays where it is. Quite weird.
C Bag, S bars here too.