Sports drinks, to be effective and do what they are meant to do, rehydrate and replace electrolytes, need to contain some sugar to help with this reaction/absorption.
The concern from a dietary perspective with most sports drinks is they are overloaded with sugar, this is due to a combination of having the glucose to help with absorption but then adding more sugar to mask the salty taste of the electrolytes. What you end up with is a carbohydrate loaded drink that is forcing unnecessary calorie intake while rehydrating.
The consumption of a highly concentrated carbohydrate solution during exercise can force the intestines to intervene to dilute this solution before absorption. The intestines do this by temporarily shifting fluids away from muscle cells which will have a negative impact on both muscle function and sweat rates because it causes a temporary degree of plasma and tissue dehydration.