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.
And are you implying that it is simply KHTML rebranded and that they have not worked on it for more than 10 years?