Because hardware can be bought with another OS installed and then reinstalled with Linux (having checked whether it's fully compatible beforehand, though that's quite okay nowadays). Most others will probably be doing that, but then you're supporting the Microsoft empire, which is what Hettes was trying to avoid.
The ethics and economics are frustrating to untangle. Without the Microsoft juggernaut, it's certainly true that we as users would have more choices. We could pay Apple $5000 for a Mac, Commodore $5000 for an Amiga, Sun $10000 for a Unix box, or pay steep prices to any number of other vendors selling incompatible boxes.
Point being, I'm not sure the economics of buying a Windows box and overwriting the hard drive are really that bad, compared to what might have happened in a world without Microsoft's standardization efforts.
Standardization would have happened without Microsoft. The progression of almost every new technology is that it becomes more standardized as it become more mature. Take operating systems as an example. At the beggining, every computer had its own OS and software written for one computer likely would not work on others. This was a problem, so eventually we came up with BIOS, which provided a standard software interface to different hardware.
Later we had a similar divergence in operating systems. Eventually Bell Labs created UNIX. Excluding windows, the UNIX specification is the standard of most modern operating systems (OS X, GNU, BSD, Solaris, ETC).
