This research paper suggests that, in general, pH, temperature, and ions don't generally significantly affect our perception of sweetness intensity. Since temperature and acidity are the two major differences between iced and hot coffee (depending on preparation), this more or less rules out the possibility of a chemical difference.
"The main finding from this three-part study was that temperature, pH, and ions had little effect on the perceived sweetness intensity of the sweeteners studied. Even when significant differences were found in the temperature study, the effects were very small."
This, to me, suggests it's either due to larger "sips" of cold coffee and more of an opportunity to taste the sweetness or because sugars dissolve less readily in the cold coffee and it's easier to perceive the sweetness when it's not in a solution.