Because the link you provided is for Surface PRO, and specific to the x86 offering.

From: http://en.wikipedia.org/wiki/Windows_8#Secure_boot

Microsoft's certification requirements eventually revealed that UEFI firmware on x86 systems must allow users to re-configure or turn off secure boot, but that this must not be possible on ARM-based systems (Windows RT).

Because then you can't complain about what a horrible world it is with Google and Apple "oppressing" customers. Some people can only complain.