While Microsoft can't punish companies for selling rival operating systems, machines with anything but Windows installed don't help PC makers meet the quotas necessary to pay Microsoft lower rates.

"Microsoft doesn't leave much room for negotiation," says Citigroup analyst Brent Thill. Nor does it have to.

What if Linux went luxe?

Hewlett-Packard could, in theory, save some money upfront by putting Linux on 10 percent of the desktop PCs it sells at retail. But that would just make the rest of its Windows desktops more expensive, because it would no longer be selling as many Windows licenses as archrival Dell, and Microsoft could legally charge it more.

HP, which recently overtook Dell the world's largest PC maker, can't afford to hand its vanquished rival any cost advantages.

In a nutshell, switching to a rival OS system will hurt more than help the bottom lines of PC makers. The straightforward economic scheme that regulators set up to keep Microsoft in check is essentially keeping the PC industry in the Windows fold - just as Redmond's brass-knuckle tactics once did.