It is illegal to ship alcohol via the USA Postal Service - why?

Where do you figure this law came from? I mean i figure they would break it all anyways but interesting law. Also, under U.S. law (49 CFR 173.29), a container that previously held a hazardous or restricted material must be treated as if it still holds hazardous or restricted materials. For example, it is illegal to ship alcohol via the U.S. Postal Service; if you reuse a box from a liquor store that once contained alcoholic products and the barcodes and labels indicate that fact, the parcel will not be accepted by the postal service because of the assumption under U.S. law noted above. http://canada.usembassy.gov/content/textonly.asp?section=can_usa&subsection1=borderissues&document=borderissues_postal_102504