It looks like a maths library, what was so special about it, and why does Apple needs to make its own?
http://floating-point-gui.de/xkcd/
A good quality math library that is optimised to take advantage of new processor extensions whilst still being correct is worth a lot.
I would imagine that there is a lack of engineering time to make this open source, rather than some nefarious purpose.
That doesn't mean it's a nice thing to do. Was this derived originally from FreeBSD code? This could be why the GPL is a good thing.
OS X's libm contains only assembly functions re-written from scratch, originally in PowerPC assembly and then in IA-32 and x86-64, to be faster, more accurate and more standards-compliant than the FreeBSD versions.
I hope that this announcement will at least result in some interest in the versions that have been released as open-source and are still available, because this is beautiful code. But I am an optimist.
(Personally I'm not sure if it's even possible to definitively say which situation is better; you can't do something scientific like releasing a library under both licenses and seeing which people use, because they would interfere - you really need two parallel universes to see the true effect of that one change)