Webkit is open-source because the people who wrote it (KDE devs) put it under the LGPL. Apple was playing catch-up, and used the best code base they could to get started. They aren't philanthropists, they are just (grudgingly) abiding by the license of the software they are building on.
It certainly used to be grudging -- originally Apple just threw a huge clump of code back to the KHTML guys, technically satisfying the LGPL, but not doing anyone much good. That's a far cry from the community they've built around the project now. I wouldn't call Apple philanthropists, but they're not simply following the license either.
They also could have rewritten that. The original KHTML code is a pretty small part (about 140k loc initially) compared to the WebKit project (about 3M loc).
Also, they only really needed to have WebCore and JavaScriptCore open but they decided to make all of WebKit open later on (all the rest under a BSD licence).
What do I know. One of the ancestor comments introduced the concept: "They aren't philanthropists, they are just (grudgingly) abiding by the license of the software they are building on."
As an attempt to answer: They can just satisfy the letter of the GPL by a code dump--which I would count as grudgingly, or they can embrace open development.
