Apple is obviously following the innovations of web app design here. People use web apps with wildly different designs all the time, and yes, there are some usability issues with that, but when consumer OSes were first being developed some decades ago, HCI researchers assumed that everything had to look exactly the same between applications or users would freak out. The web never had a single HIG, so we found out that users are much more tolerant of differences, or at least they are now.

As web apps become more important, the notion of an OS HIG needs to evolve. Obviously an app needs internal consistency, but how consistent do two apps need to be between each other? An apt comparison is the world of typography: the rules for good text layout don't specify one font for every single book, we can handle the fact that different books use different fonts. But there are still rules for good typography, they're just more general than "All uppercase G's should look like this: G." This is the direction that HIGs would need to go, but whether this is a good thing is debatable. For whatever reason, I have a hundred or so books on my bookshelf and basically all of them have very good to excellent typography. Web sites and apps have nowhere near that level of consistent quality. Native apps are better, but now we're going to start to see more of the web's no-rules design norms applied there, which probably means a few really gorgeous apps and a whole lot of crap.