Explained by a professor of food science:
What really piqued my curiosity was the inclusion of “sugar”... Including only about four one-hundredths of a percent (i.e., 0.04%) of this additive protects or stabilizes the potassium iodide. If no glucose was present, the potassium iodide would eventually break down into its component parts - namely potassium and iodide ions. The iodide ions could then combine to form iodine that could actually vaporize and leave the salt.Image cropped for size and color-corrected from the original.