Drew McCormack over at MacResearch speculates as to why in Grand Central Now Open to All | MacResearch:
There could be one last reason why Apple has taken this step: they want to use Grand Central to push the adoption of other technologies, in particular, blocks. Blocks are an extension to C which form the basis of Grand Central Dispatch. Having your operating system based on a non-standard language is not a good position to be in, and Apple would surely like to see blocks incorporated into the C language. By offering Grand Central to the broader programming community, they may be hoping it will catch on, and make the argument for incorporating blocks in the C standard that much stronger.
The next step is for this code to be picked up in a Linux Distribution. Having simple concurrent programming, integrated tightly with C code (and C code derivatives) would be a boon to many industries looking to exploit multicore.