It's been long since, so I don't remember all the details, but it was a government anti-piracy regulation at that time in some countries that you either ship with another OS or you pay MS because practically all computers sold without an OS were used with some pirated MS OS.

Such laws existed and may still exist, I remember Germany had some tax on CD-RW disc sales because "they were used also for piracy". All it took was "lobby" at the right level of government.