Actually, WebKit was a fork of KHTML, correct, but it is licensed under the terms of LGPL. One extra letter, but a world of difference. This is why they had to open source KHTML, but they could get away with not open sourcing Safari (if KHTML was licensed under the GPL, any app that linked against the library would need to be licensed under the GPL, too.)

Let's not forget that webkit when it was released was little more then a code dump - one big patch that had to be applied to KHTML.