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.
Also "they didn't have a choice" sort of ignores that they did have the choice of not using khtml and wouldn't have, if its license could've been problematic for them.
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.