Apple does contribute beyond what is required. They are not required to post their updates to LLVM+clang, but they do. launchd was considered for Ubuntu before they licensed it under Apache (it was under APSL at the time). They didn't have to hire the CUPS dev to work on it. If you go through Apple's OSS listing [1], if it has the APSL license, that's something they released themselves.
> They are not required to post their updates to LLVM+clang, but they do
You open-source the projects you don't consider strategic assets and/or projects that would cost too much or take too long to develop internally. Open-sourcing is a tool - a development model. That's also why NeXT based its OS on Mach and BSD (and licensed Display PostScipt) - because that shortened their time to market and allowed them to build a computer will a full operating system with enough competitive differentiation to survive for some time (and be resurrected later in the form of OSX)
NeXT was very reluctant in sharing their GCC front-end but was eventually forced to comply with the GPL. Apple has no such obligations with LLVM-clang.
[1] http://opensource.apple.com/