presumably because a) it still allows the source code to be available and used for the 'permitted purposes' (i.e. anything that's not directly competing), and b) it represents a concrete commitment to open up, not just a pinkie promise (even if they were to have a license or contract which promised it, it would not be as easy to rely on as actually having the source code published. Companies have reneged on such promises before).

And yeah, by my reading essentially people can contribute code or publish patches (with just a plain MIT license in principle), just the original and derivatives still can't be used for non-permitted purposes until the timer is up.