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 indisputable that Apple seeks large amounts of control over its products. So it’s a reasonable question to ask whether Apple sees the web itself, which they have no control over, as a problem. I don’t think that’s the case at all, though. The web, as a whole, is arguably the single most entrenched computer technology ever created. So where Apple seeks control with regard to the web is in the technology to render it — HTML, CSS, JavaScript. No one can tell them what to do with WebKit; they wait for no one to shape and bend WebKit to suit their needs.
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.
WebKit is the finest Web engine in the world, faster and more compliant than Gecko at rendering markup. And for JavaScript, WebKit developers have their choice of two amazing interpreters—Apple's Nitro Engine and Google's V8—both of which are open source.
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.