The powdered sugar (confectioner's sugar) you buy at the store has corn starch in it to keep it from clumping. It's been said the the corn starch isn't good for the bees. My mentor recommended putting granulated sugar into a food processor an pulverizing it. That works, but you have to use it right away or it will form into one solid mass.
In my experience, the store-bought confectioner's sugar hasn't hurt the bees. It's easy, convenient and reduces my mite counts.
In a perfect world, maybe I wouldn't give my bees something with corn starch in it, and maybe I wouldn't let my children eat Captain Crunch for breakfast....