I believe all the items I read about seting up CO2 chambers HAVE said not to use it for anything larger than a rat. I'm not sure why, but I do remember reading that.
I understand them getting it to the side before shooting it. Besides, this way they took care of it, you didn't get into any trouble over it, and the raccoon is finally dead and out of pain.
Animal Control should be jumped on for letting i stay there, when it was obvious the animal was in pain and damaged.