Making ice from carbonated water
There should be no issue with putting carbonated water in plastic bags and freezing them (considering they're clean and sterile, of course). I imagine the benefit of freezing carbonated water instead of regular clean water has to do with the fact that there is much more dissolved gas in carbonated water. As the water freezes, the gas is expelled from the ice (only the water molecules will solidify), creating many pockets inside the solid ice which would weaken its structural integrity. This would likely make it much easier to shave or break into small pieces for a snow cone. I've never frozen carbonated water myself though. You should be a bit careful because as the carbonated water freezes, the carbon dioxide it releases will create pressure in a closed container, so you may end up with torn plastic bags, ruptured bottles, or a damaged freezer in the worst case.