> You can't write or run arbitrary code on it, you can't swap the OS, you can't even SEE the OS.

You can write and run arbitrary code on it, you can swap the OS, and you can even rebuild the firmware from source and flash it yourself (though at that point you will void the warranty since the firmware is in a position to cause hardware damage, but not for any of the other things). There is a local terminal emulator, and when you turn on developer mode you can have a local unix shell with mount and write access to removable disks, so you can image install media for an operating system. The boot firmware comes with SeaBIOS by default for the reason that Google (for whatever reason) wants you to be perfectly able to install bios-based operating systems as long as they're compatible with the hardware. If it's Linux, then any downstream hardware drivers involved are packaged in google's public kernel repositories, and if it's a userspace component, it's in the ChromiumOS repositories, where you can pull and build the entire operating system (aside from the pepper plugins for digital restrictions management and flash).

Seriously, try doing many of these things with a typical off-the-shelf Windows or Apple laptop and get back to me. On Windows laptops you need to jump through hoops to disable firmware features which prevent you from installing new operating systems, on Chromebooks it's well documented and has no effect on warranty or your ability to roll back the changes. Most (all?) windows laptops have no vendor-provided open source firmware, and most of them don't have even a third party one which works. To write an install disk on Windows, you need to install a third-party image burning application, and there's no guarantee that the firmware will even let you boot it. Many windows laptop manufacturers will either inconvenience or flat-out deny warranty claimants who have replaced the operating system on their laptop. Microsoft does not publish the source code of the NT kernel. Most Windows laptops contain at least one piece of hardware which is not adequately documented such that somebody could write a driver for it without reverse engineering.

Apple laptops are in some ways better, in some ways worse here. On the one hand, the firmware does not explicitly prevent you from booting third-party images, on the other hand, the firmware and hardware have undocumented behaviour which makes it exceedingly difficult to get most new operating systems to reliably boot and install on it. Usually even if yo u do manage to install and boot something, some important piece of hardware (the wireless chip, the display output multiplexer, the webcam, the touchpad) is either slightly or completely different from a documented counterpart, and takes months or years to be supported by anything except OS X and the first-party drivers on Windows. Darwin releases are usually eventually open source, Apple's compilers are partially open source, but also include several proprietary components. Most of the operating system libraries which weren't already available when they started are proprietary. Though on the plus side, you don't really need anything extra to write a boot disk for a third-party operating system. On the down, there is no provided firmware source code, and the system is sufficiently underdocumented that it's unlikely anyone will ever write an alternative firmware image for your given Apple laptop model.