I also thought the exact same thing, but I know that encoding stuff is so complex that I may be missing something. For what its worth, I believe Cocoa does things this way: a string is an abstract object of unicode code points, when its time to read or write you must choose an encoding.
The Java and .NET infrastructures make that choice for you too. I guess I wasn't there when Java first got started, it's possible that there was a backlash[1] against this decision by the people who are using encodings that aren't subsets of unicode. I don't get to see many "enterprise" systems, but I get the impression those applications where it's important to use some kind of not-unicode are sufficiently rare that they're best served by specialty libraries. You can't make everyone happy with one solution, but if a simple solution works for 99%, then I'd call that a roaring success.
[1] a backlash other than the OMG-2-bytes-per-ASCII-character hysteria, which is irrelevant for these purposes, they could just as well have chosen UTF-8
