At a guess - they're likely selling those laptops at a loss and making the money back on (hopefully) service contracts or (probably) selling your data. As soon as you install a custom OS they won't support you (so you won't buy support) and they won't be able to sell your data.

Most Chromebook's firmware is Coreboot, but it's running a Depthcharge payload instead of UEFI (or BIOS or whatever). Mr. Chromebox maintains UEFI Coreboot payloads and install tools for a wide variety of (x86) Chromebooks, which can be used to flash a normal UEFI payload and boot normal OSes. It's strictly possible to boot normal Linux systems on a the Depthcharge payload modern Chromebooks use, but uh... here's the gentoo wiki on it, it's a substantial pain in the ass.