Apple sells both the hardware and the OS that runs on it, they own the whole stack and thus can do pretty much what they want with it. Even so I don't think there are any problems installing a Linux distro on any Mac, and Windows is directly supported by Apple via bootcamp.
Microsoft only owns the OS part of the stack, and this move, combined with some shady back room dealing or just relying on plain old OEM incompetence, could potentially prevent you booting any other OS on hardware they don't own or manufacture themselves.
> I don't think there are any problems installing a Linux distro on any Mac
None whatsoever. In at least Ubuntu's case, you need do little more on modern Intel Macs than insert the CD and reboot. If you want to keep OS X around (and have a few other goodies), install rEFIt first for better partition and boot management.
In some ways, dual-booting Linux and OS X is less problematic than Linux and Windows has ever been.
Microsoft only owns the OS part of the stack, and this move, combined with some shady back room dealing or just relying on plain old OEM incompetence, could potentially prevent you booting any other OS on hardware they don't own or manufacture themselves.