What you're referring to are called "80% lowers" and they're the part of an AR-15 rifle that contain the serial number. AR-15s come in a lot of parts, so the "lower" is considered the actual gun that has a serial number and requires a license. So manufacturers are selling machined lowers that are 80% complete, considering this as just a piece of metal, not a gun. The ATF has already raided at least one maker of these parts and taken their customer list.
Personally I think it's a grey area. At what point do you go from "it's a bunch of metal, screws, and springs" to "firearm". It's the sand hill paradox. And if you want to stay out of jail, stay away from these. All it takes is a zealous DA who decides it's a violation of federal fireams law and suddenly your life is hell.
There are other 80% complete frames available and have been for a long time. The most popular are the AR-15 and 1911. It seems like the line is fairly well established. There are other issues like felons and other people who can't have a gun using this to get around the background checks that can lead to raids and questions about the makers of these billets but in general if you can own a gun you're perfectly safe to buy an 80% billet and machine it out yourself.
Are you talking about the CNC shop that was trying to act like just starting or touching the machine was enough to count as their customers manufacturing the lower instead of the company? That's a whole other issue with just what does it mean for a person to have manufactured the weapon and I'm pretty comfortable saying that company was well on the wrong side of that line.
For most 80%s they're pretty clearly not a gun because there's absolutely no way to use this [1] without modification and assemble it into a gun. This basic design has been around for a looong time.
It's not that much of a grey area. The ATF has published regulations that require the fire control control pocket be created before a lower receiver is legally considered a firearm. The people who have gotten in trouble here have created lowers where the fire control pocket has been started in some way. In one example, a company created a "biscuit" of one color plastic in the shape that needed to be milled out and then shot the rest of the lower around that biscuit. The ATF argued that the fire control pocket was created in this process and the end user was just removing a plug that was inserted into that pocket. In other cases, the manufacturers made index markings showing where to mill things out.
If you stick to an 80% where the fire control pocket has not been started and you select a lower that requires a separate jig to mill, you'll be fine.
Personally I think it's a grey area. At what point do you go from "it's a bunch of metal, screws, and springs" to "firearm". It's the sand hill paradox. And if you want to stay out of jail, stay away from these. All it takes is a zealous DA who decides it's a violation of federal fireams law and suddenly your life is hell.