Meta is a place for discussing the Stack Overflow trilogy. The people best suited to the manner of discussion that occurs are the active community that participate in the trilogy. So it naturally follows that there is a level of feature that restricts participation in talking about the trilogy, to those who actually are a part of it.
Registration is fairly simplistic. If you want to really voice your opinion about Stack Overflow, then you should make it known that you actually care about the community. And if you cannot be bothered to actually register, then it says lengths about how little you actually care about being part of the community. And there's no real benefit to discussion the community with someone who (apparently) doesn't even care how the community ends up.