> they do test that their hardware works with Linux.
They not only do that, they pre-install Linux and support it on their hardware. System76's approach is to create a special driver to fix up any lingering bugs (on the Gazelle, it says that it's adding screen brightness hotkeys and the sdcard reader), so they only ship with Ubuntu. ZaReason's approach is to ship any Linux on their machines, so if the cardreader doesn't work they just won't advertise the cardreader. So you may get a bonus upgrade in a release or two when the cardreader support lands in your distro.
In practice, I think I prefer System76's approach. My Terra HD had a lid state detection issue (tracking it down with mjg59's help, it may have been an issue with the Intel driver, but Intel wasn't forthcoming with the docs necessary to get it fixed). Whereas so far my Sytem76 Gazelle has been perfect. Of course, this is also comparing a late-edition netbook to a full i7 laptop so there might well be a hardware quality issue innate to the respective ODM markets. As others have said, they don't build their own hardware. Neither does Dell or HP or Lenovo for that matter. Instead, they all buy parts from ODMs and assemble them to the final product. So they are susceptible to the underlying ODM market--they don't have the clout of Microsoft or Apple, particularly because people insist on buying Windows laptops and putting Windows on them.