WebCore is based on KHTML, and is thereby licensed user LGPL: it is open source, and due to that license choice Apple is stuck and can't fix it. However, WebKit, a library that sits on top of WebCore initially by Apple, and where a lot of the "little touches" (such as multitouch JavaScript events) goes, is under modified BSD, and Apple often does not release code for it back to the community; hell: even for WebCore they sometimes play games distributing binary object files for parts they want to redact.
That Apple used those resources to steward a successful open-source project themselves rather than just sending patches back to KDE makes the point, though.