Sugar is able to melt ice because it disrupts the equilibrium of the water molecules and causes the freezing process to slow down, resulting in conditions that favor the melting process over the freezing process. The addition of a foreign molecule, such as sugar, results in a lower freezing point.
This phenomena is known as a freezing point depression and occurs when outside molecules are added. Sugar, salt and alcohol all cause this process to take place. When a mole of foreign molecules is added to a kilogram of water, this decreases the freezing point approximately 1.7 to 1.9 degrees Celsius. The lower freezing point makes it difficult for the water to refreeze.