Why does Apple keep info on it's pre-released Operating Systems under NDA?

As a hobbyist Mac programmer, I have access to pre-release software such as the just announced 10.8 Mountain Lion Mac OS X.

The problem is that all of the developer-related goodness is locked under NDA, so I can’t ask questions, or benefit from the questions/answers of others on sites like stackoverflow.com, or on the many mailing lists, etc. Apple does provide access to it’s registered developers on a secure Apple forum where these things can legally be talked about, but the problem is that Apple’s secure forum isn’t very busy, many questions go unanswered, and there isn’t the rich discussion that we see in other places.

Anyone can sign up to be part of the Mac Developer Program for $100/year (there’s also a free account, but you don’t get access to pre-release stuff with that). As a result, the NDA isn’t preventing competitors from accessing any of this information, as it’s trivial for them to make an account and look up anything they want.

So my question is why does Apple do this? There must be some legal necessity, because having the people developing software for your platform and helping test the OS and discover bugs being able to freely communicate is very important to that process. I’ve never understood this, and it’s been going on for many years.