Nothing stops anyone from using the APIs, I'm talking about a working implementation.
OpenGL is supported on pretty much all available platforms and has a standard implementation on them: Windows has opengl32.dll, Linux has Mesa3d, and Apple also has a default implementation.
I guess the point I'm trying to make here that an API is worthless without an implementation: the library containing the actual functionality. What are you loading if you don't have a IHV implementation available? Nothing. Just like OpenGL, OpenCL will need a default, software, implementation supported on all platforms.
And I can promise you that Microsoft will not be jumping on this OpenCL bandwagon (providing a platform default software implementation) with their development Direct3D Compute Shaders and the fact that Microsoft is no longer a Khronos partner. If they do in the next version of Windows I'll be very pleasantly surprised.