What makes alcohol-free beer sweet?
Beer is made from malted cereal grains. These can be corn but more commonly malted barley. The malting process turns the starch in the grain into sugars then the fermentation turns the sugars into beer. To get a strong dry beer, first start with a sweet malt and the ferment until as much sugar as possible becomes alcohol. For a non alcoholic beer, you need to remove the alcohol so you start with a low sugar malt and ferment just long enough to get the flavour but not too much alcohol. Since the fermentation is short, even through the initial sugar content might be low, there is sugar left in the beer thus giving you a sweet taste.