Why not? This is easily answered.
Secure Boot does not mean "secure for you". It means secure for Microsoft.
The advertised "feature" of anti-virus, anti-malware is a strawman. I don't get why any hardware manufacturer would be so unbelievably greedy and implement this just to get a stupid "Works with Windows 8" sticker, but obviously I am just too stupid to get it, because it will probably happen.
I really, really hope something will happen, that prevents the whole secure boot thing with MS certificates. I don't know. Maybe the European High Court (or whatever its called) does something about "secure boot"
There would be a really easy way of implementing secure boot without any certificates from a special vendor. Just securely hash the boot sector at first boot. If the has changes, ask the user to verify the change or rebuild the old boot sector. This way you cannot change the boot sector without the user knowing it. Easy as this and as secure as the so called "secure boot". And as a bonus you still can run any software YOU want.