Gruber, in discussing the current blogstorm over Apple's continued non-support for Adobe Flash in iPhone OS, touches on why Apple—who certainly have no problem with proprietary technology so long as it's theirs—are being such champs about keeping core internet technologies like WebKit free and open source:
It's tempting to reduce everything to some kind of strategic play, but I think the reason Apple open-sources their technology, and embraces open formats like AAC, H.264 and ePub, is because that's just how things are done on the Web. Microsoft tried building a wholly proprietary Web back in the IE monopoly days, and that just left them sitting ducks for competitors offering faster, more open alternatives.
Standards evolve, and they do so in the open, and Apple can't control that. WebKit is Apple's way of ensuring that whenever the Web goes next, they will be there.