They do contain natural sugar but they have a low / medium glycemic index score because they also contain a good amountof fiber, which slows the release of the carbohydrates and gives you sustained energy rather spiking your blood sugar crazy high.
Whereas humans can do rather well on diets high in (complex) carbohydrates, cats require large amountsof protein in order to stay lean, fit, and healthy.
It means ensuring that dogs are fed diets with real meat and balanced amountsof protein, fat and carbohydratesrather than diets high in carbohydrates and fillers.