John, I think we're looking at the Staples announcement differently.
They have no plans to have these printers in their stores, any time soon. They'll be at centralized locations.
You upload the model file(s).
They print it.
Then it's shipped to your door, or your chosen Staples location.
If so... there's no way to be present when it's printed, to skirt any ATF regs about manufacturing a receiver*.
Printed by Staples, milled by eMachineShop, or made by the machinist down the road while you're at home... it still can't be a finished receiver, for them to legally produce it. (If they'll do it at all.)
*(I've never seen an official ruling or opinion letter stating that it is, in fact, legal for some one else to produce the receiver, so long as you're present at the time. I've seen many references to it for suppressor parts and AR receivers, but never an official statement. ...thus, my use of "skirting" ATF regs. If anyone does have a link to back that concept up, it would be good to see.)
"Such is the strange way that man works -- first he virtually destroys a species and then does everything in his power to restore it."