Windows 8 competes with Windows 7 and they have to allow users to upgrade with an old PC. It would be stupid to implement an OS that requires a Secure Boot mode, because it would mean that mean that users would have to buy new hardware.
You don't understand binary signing or secure boot, do you? Windows 8 binaries will be signed. Signed binaries can run anywhere. It's only when hardware or the BIOS checks the signature before allowing code to execute that binary signing matters, and then only for binaries that are unsigned. Nobody has suggested that Windows 8 won't run on computers without UEFI secure boot. Microsoft has stated that computers which ship with the "Microsoft Windows 8 Certified" sticker must have secure boot enabled by default and ship with MS's public key such that the UEFI system can check the signature before allowing the system to boot.
The concern is not that Windows 8 won't run on old computers (it will.) The concern is that hardware manufactures will ship computers will ship UEFI systems that don't allow the consumer to add their own public keys or a means to disable secure boot (such as the hardware switch Google requires on ChromeBooks). It's not a concern that MS is requiring they don't allow secure boot to be disabled (and that's why its not an antitrust concern), but that OEMs will not allow the user to disable it.
Garret/RedHat has apparently confirmed that some OEMs intend to do just that. And it's in their interest. They already try to require that you boot windows to run their goofy utility to prove your CD-ROM is non-functioning. With secure boot required, the OEM (Dell, HP, Acer) knows the computer will only run Windows, so they won't have to train their employees about how to handle cases where the consumer is running an unsupported configuration and unable to run the tool.
It would be a stupid business decision especially when over 95% of consumers prefer Windows over Linux anyways. There is little to gain for Microsoft and a lot to lose
Indeed it would. And that's why they're not doing whatever it is you thought secure boot meant, nor are they requiring that OEMs only allow Windows to boot. However, they also aren't following Google's lead and requiring OEMs include a way to boot anything other than Windows. And some OEMs might take that cheaper route. Indeed, RedHat says some intend to.