It doesn't need to in most cases. If you fill the fish bag 1/3 water 2/3 air (enough water to cover the fish completely no matter which direction the bag is turned), it'll have enough air to survive through priority shipping.
Breather bags have their place, I guess. One benefit is there is no sloshing, so it tends to be easier on the fish/inverts being shipped. However, it does have several drawbacks as well. First is that they leak sometimes. It's fairly rare, but there are lots of accounts of it, especially with spiny fish. You could double bag with them, but it decreases the o2/co2 exchange greatly, plus it drives the costs way up. Second, they are expensive. Last I looked, it was around .50 per bag for the little bags.
People will say 'oh well I don't mind spending more to make sure they are safe', to that I say 'pay for express shipping, then'
I do know of at least one shipper that uses breather bags and he has some sort of sealer that he uses to seal them off in little sections. Literally, individually bagged in little 2" baggies. It works really good, but would require a sealer and skills to do it safely and efficiently.
