Marketing Defeats Usability
There's no technical reason why the start button can't be optional - in fact it would probably be easier to retain the start button functionality in parallel to the start screen. It's all just marketing.
The compulsory Metro interface, especially its substitution of the Start menu, is just to get us all familiar with Metro. It's irritating and far less convenient than the Start button, but it means most computer users will become used to the Metro "look and feel", as well as its branding.
So then we'll find WinPhones and WinTabs quite familiar looking. The Metro interface itself is great, but largely unknown. So this exercise is to advertise it to the millions of new Windows 8 users coming up, so that the phones and tabs will sell.
They reason that it's worth pissing off power users if they can expand Windows sales at the lower end.