shouldn't I be able to just whirl some in my food processer until it is a powder and save a few bucks?
Sure. The icing (powdered) confectioners sugar has some small amount of corn starch in it which isn't particularly digestable for the bees but apparently isn't a problem during flying weather.
I tried grinding some up in a blender, it took a long time to get fine enough. I suspect what I saved in money I took off the lifetime of my blender.