The milk that comes in these types of packages is also heated to a higher temperature in the pasteurization process, which kills more bacteria. This gives the milk a longer self life before it goes bad. Ultra High-Temperature Pasteurization combined with the aseptic packaging gives the milk a 6 month shelf life without refrigeration. This is the way milk is commonly sold in many other countries.
As a PP mentioned, some organic milk (Horizons) is sold this way. I buy this type of milk to send in my son's lunches. We refrigerate it and use ice packs in his lunch bag so that the milk will be cold like he is used to, but I like not having to worry about the milk going bad if it does happen to get too warm.