there are a couple of issues with confectioner's sugar wrt coffee shops.
1. People rarely pay attention to what they are doing when pouring sugar and the last thing you want to do is pester your clientelle with reminders like...please do not let the shaker touch the coffee.
2. Knowing where you are...powdered sugar will not remain powdered long in the mugginess that is Central Texas(especially next to a large body of water)
3. Health inspectors frown on it(at least in Austin)
4. Most shops already carry three or four forms of sweetener(raw, bleached, cubes, splenda, the pink cancer packets, the blue cancer packets, honey, etc) and few actually have room to store more.
However...If you frequent one shop regularly you should go ahead and just ask them to keep some behind the bar for you. No good barista will turn you down especially if you bring your own.