Apple has a strong incentive to make sure that people using OSX have a great experience. To make sure that OSX can browse the web well they need to make sure that Safari is viewed as a mainstream browser, and that it's easy for developers to test websites on it without having to go buy a Mac.

So by releasing it on Windows they can (a) gain marketshare so it's taken more seriously, and (b) make sure developers who use Windows will test with it.

As for why they didn't just use Gecko (Firefox's rendering engine), Apple also wants to be certain that websites look great on the iPhone/iPad. And my understanding is that Gecko is an absolute monster to get working well on mobile devices. Plus by having their own rendering engine they get to be certain that there'll be no problems adding features needed for touch-based apps to it.