I was reading a book on Microsoft's history and they did that back in the DOS days: any IBM PC clone that was sold had to pay for the MS-DOS license regardless of if the PC would be running MS-DOS or not. So unless someone explicitly asked for DR-DOS (and was fine to pay for both the MS-DOS and the DR-DOS license since the former would need to be paid anyway), manufacturers just used MS-DOS because it was "cheaper" (and with MS-DOS being the "standard" it isn't like they could afford to not license it).
