I think since they eat so much junk, the sickly-sweet stuff just tastes normal to them, since they're used to it.    I noticed a similar thing happen to me first-hand.  I used to put sugar in tea, but if I had a few cups per day, thats a lot of spoonfuls of sugar.  I cut back to the point where I had no sugar in tea, and I like it that way now, where before, if we were out of sugar, the tea tasted really bland.  I had a cup a few months ago, someone put a spoonful of sugar in by accident.  It tasted really sickly to me, and that was less sugar than I used to have, back when I didn't care.