I don't mean to discredit Apple's contributions, but your comments seem to suggest to me that WebKit remains open source for benevolent reasons.
They built the project on KHTML so they wouldn't have to start from scratch. And because KHTML is under the LGPL, WebKit will have to remain open source unless they removed all KHTML-related code, which I'm sure is not worth the investment.
(Yes, clearly Google has contributed a lot of open source code too).