The same law they use to define a "finished" receiver as a controlled item. My guess (I am neither a law enforcement officer nor a lawyer) is that it would be part of
http://www.ecfr.gov/cgi-bin/text-idx?SID=70394195a3edf623eba... given that covers most transportation.
That said, my father maintained a federal firearms license for 30+ years in order to be able to send and receive guns that he had engraved. In his dealings with the ATF he noted you could agree with them or take them to court but there really wasn't an option for suggesting directly they didn't have the right to do what ever it was they were doing.
I expect that should the ATF decide that these things should not be allowed to be transported, they will do so under some rationale based on Title 27. Whether or not it is legal for them to do so, is something the courts would have to litigate, and something which neither FedEx nor UPS would be likely to do (DD could as they would have standing as the party being harmed).
In the Ars coverage (http://arstechnica.com/tech-policy/2015/02/fedex-wont-ship-t...) the ATF was quoted as saying:
As a way to get around that law, manufacturers can make a product that isn’t technically a gun, but gets as close to the line as possible. The Ghost Gunner CNC mill requires that the buyer provide their own semi-finished lower, which is sometimes dubbed an "80 percent lower."
"The term 80 percent receiver comes from the seller, that’s not a term that we use," a spokesman from the Bureau of Alcohol, Tobacco, and Firearms (ATF) told Ars last year.
"That’s a term from the seller or creator of the device that it’s 80 percent of a finished receiver, therefore it is legal. That’s not a determination we make."
In short, in the ATF’s view, either something is a firearm or it isn’t—and if it isn’t, it can be sold without restriction.
But in January 2015, the agency released ATF Rule 2015-1 and clarified that a gunsmith or machine shop cannot use its tool to refine an 80 percent to make it a suitable firearm, but the ruling does not affect CNCs owned by individuals.
"Nothing in this ruling affects the ability of individuals to make their own firearms for personal use, as allowed by law," Ginger Colbrun, an ATF spokeswoman, told Ars by e-mail on Tuesday.