So why aren't there any dual-boot computers for sale? The answer lies in the nature of the relationship Microsoft maintains with hardware vendors. More specifically, in the "Windows License" agreed to by hardware vendors who want to include Windows on the computers they sell. This is not the license you pretend to read and click "I Accept" when installing Windows. This license is not available online. This is a confidential license, seen only by Microsoft and computer vendors. You and I can't read the license because Microsoft classifies it as a "trade secret." The license specifies that any machine which includes a Microsoft operating system must not also offer a nonMicrosoft operating system as a boot option. In other words, a computer that offers to boot into Windows upon startup cannot also offer to boot into BeOS or Linux. The hardware vendor does not get to choose which OSes to install on the machines they sell--Microsoft does.
"Must not?" What, does Microsoft hold a gun to the vendor's head? Not quite, but that wouldn't be a hyperbolic metaphor. Instead, Microsoft threatens to revoke the vendor's license to include Windows on the machine if the bootloader license is violated. Because the world runs on Windows, no hardware vendor can afford to ship machines that don't include Windows alongside whatever alternative they might want to offer.