WebKit started as a fork from khtml (from kde). Apple didn't have a choice about open sourcing Webkit because they started with GPL2 code. They would have kept it closed source if they could, but that would have taken a few years of development, while they could start with khtml and get a great browser in just a few months.

I expect in a few years (5-10) Apple will do an audit, discover that only a small amount of code isn't written by Apple, and replace that with inhouse code and then relicense to something propitiatory. Time will tell.