KHTML/KJS was LGPL, so they had to release it. LLVM and Clang is a better argument, until you realize that Apple hired on the guy from the project after their GCC grand plans collapsed - the only visible reason they shifted strategies was to keep from having to open source components of their proprietary IDE XCode.

It's very well established that Apple is allergic to the GPL, and is willing to pay good money to avoid dealing with it whenever possible.