The code is. I don't think the hardware itself is QAed in the US.

It definitely is. Huge testing facilities, radio testing, durability, temperature cycling.

All done by Apple in the US.

Do you know if they QA every device or just spot checks for quality of batches? I'd also guess this is only for US-sold devices as it'd be a bit prohibitive to ship them all from China to the US for QA and then ship them to Europe for sale. Is that the case?

I work in manufacturing test, so I can speak to this.

There's a difference between design/development test and manufacturing test. Thermal, drop, vibration, interference, etc. tests are design tests: they verify that units built according to spec are sufficiently reliable. None of the devices you buy have undergone these tests, because the tests are destructive. You wouldn't want to buy an iPhone that had been baked in an oven, or dropped on a concrete floor. It's best to perform these tests near where development happens, to enable fast iteration and easily involve the hardware engineers.

Manufacturing tests verify that individual units have been built according to spec. For example, that there's no electrical shorts, missing connections, cosmetic defects, etc. All of the devices you buy have undergone a large battery of manufacturing tests, which exercised all of the hardware features of the device. These tests are performed at the factory, because it's better to catch these failures early in the assembly process. For example, if a camera module is busted, you want to know before it's installed into an iPhone.

Of course Apple isn't unique in this way. This is just how manufacturing is done.

Thanks for the detailed response. I wasn't as aware of all that as I should be. I'd wager it varies based on the level of the brand, though. So, you could expect this with something like Apple or Samsung or HTC, but lesser brands may not even test their wares before shipping them. My recent purchases of a Monoprice keyboard and network adapter, for instance, both of which were DOA with obvious manufacturing defects.

Only the devices sold in US.