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Webkits performance results in a superior user experience over other browsers. The last thing Apple would want is for their competitors to be apple to provide comparable performance, it is just something apple has to live with.
LLVM and CLANG began as university research projects, had they originated with apple they might note be open source.
Even if they had originated at apple, they do not provide any benefit that is visible to the end-user, so from the Apple perspective they don't lose anything by sharing the source code.