Apple makes the hardware (indirectly). They create the device right from hardware design through to OS and UI. Microsoft only creates the OS. They are a software company except for Xbox and a failed Zune.

I believe that this also relates to market share. MS has a potentially huge impact if they become the dominant tablet OS (this is still remains to be seen).

If Microsoft made the tablet (hardware) I would be more okay with the UEFI Secure Boot. But they don't and I do not think they should dictate that level of control to the OEM (IMHO). The reason - b/c if they become the the dominant tablet OS, it puts them in a position where they could influence the OEM not to provide alternative operating systems on their devices.