If I were to guess, I would say that the reason they don't want it pressurized with CO2 is because it isn't designed as a CO2 extinguisher. CO2 extinguishers are designed with different outputs and such...I don't see any problem with using CO2 in it to just push a couple gallons of beer. That's just a guess, for god's sake don't use it if you aren't comfortable. I may give it a go this week!
