A better question is, “Why should it matter?” I don’t think you’ve sufficiently made a case for why having a similar look-and-feel to a rival OS is a bad thing.
It’s obvious that they’re trying to take advantage of UI familiarity, which is a no-brainer if your goal is to get more people using your distro (especially casual users).
There’s no point. Last I saw there weren’t even any gconf hooks for the new GDM, so anything they’d do would be hacky and wouldn’t be relevant in a short while anyway with 3.0 coming out. There are plenty of other more worthwhile things on which they can focus their resources. Yes, this includes branding.
If you really want a customizable login manager, you can compile GDM 2.27 or before, or better yet find someone with who has already done so and has a PPA available. Or use KDM to load your GNOME session, there are a lot of nice Ubuntu themes for KDM on kde-look.org.