You're correct, it is not a BATF thing.
But, the reason you can ship LOADED ammo, is because it's a totally different program. Put the red tag on and off it goes.
Now, to ship powder or primers through UPS, you have to take their training for Haz Mat, and then set up an account. I called them, so that's straight from UPS. If you can dig around and find something to the contrary, please post it.
Also, it's against the law to ship powder, primers, or loaded ammo through USPS. Don't do it !!
Yes I'm aware no USPS. I did go to UPS website and it was like trying to read and understand the IRS tax code. A few things I did come away with is that you have to be a "contract" shipper which I would guess includes the training you refer to. Another VERY important fact I noted was that "bull semen packaged in dry ice or liquefied gas is considered HAZMAT"
Unbelievable the BS!