Apple's recent release of Safari on Windows puzzled and disappointed everyone who was following the WWDC keynote last week. That was the "one more thing"? A Windows application? Why would Apple release Safari on Windows?

In Diggnation episode 102, Kevin Rose echoed the opinion of many bloggers: since the iPhone runs Safari, this is a way for Windows developers to simulate the iPhone's rendering and develop iPhone web applications.

But that's not why.

Daring Fireball got a big part of it:

It's not widely publicized, but those integrated search bars in web browser toolbars are revenue generators. When you do a Google search from Safari’s toolbar, Google pays Apple a portion of the ad revenue from the resulting page.

Normally, this wouldn't be much of a reason to justify the huge development and maintenance costs of porting Safari (and the OS X font renderer, and much of Quartz) to Windows. That's a massive undertaking, and hardly anyone would choose to use Safari instead of Internet Explorer or Firefox.

In the Diggnation discussion, Kevin Rose almost stumbled onto the justification, and the real reason why Safari on Windows now exists. He mentioned that Safari and IE are primarily used by people who don't choose their browser and use the default on their respective operating systems, while people who are willing to choose their browser will usually choose Firefox.

That's exactly right. And since the majority of PCs are Windows, and the majority of users never change defaults, Internet Explorer has the largest marketshare.

But Apple has an unfair advantage over Firefox on Windows: it can change the defaults when it installs iTunes.

Apple already requires QuickTime to be installed with iTunes, which is required to use every iPod and iPhone ever sold. As of April, Apple had sold 100 million iPods, and the iPhone is probably going to be a big hit with at least a few million sales likely within a year. Even accounting for out-of-commission iPods, users who ignore software updates, and existing Mac users, that's still a quick way for Apple to capture 20-50 million Safari users whenever they want with a simple iTunes update.

Mozilla thinks that Firefox has 75-100 million users, by comparison.

Apple is about to effectively wedge themselves into a major browser marketshare position, and get all of the benefits of that (search affiliate revenue, leveraging other products, choosing the default homepage, etc.) with the side effect of putting the Apple brand in front of people more often and selling more Apple products.

It's a genius business move, and it has nothing to do with the iPhone.