Sugar is what feeds the scoby. Often the kombucha mother doesn't eat up all the sugar leaving some residual sugar behind. The Good Guys kombucha has less than 5 g of sugar / 100ml while fruit juices and sodas often have 10 - 20 g of sugar / 100ml. In kombucha sugar is not added as a sweetener but for start the fermentation which results in healthy end products, such as organic acids and good bacteria.