Matt seems to me to have the answer for why Apple released Safari on Windows:

There is only one reason Apple released Safari for Windows and that is because they were forced to. By making the iPhone development environment web based apps only they had to release Safari for Windows so people could test their applications.”

As Justin Williams points out:

With the announcement of the iPhone SDK being based on Safari, Apple was forced to put Safari on Windows, so they could attract Windows developers to build iPhone applications. By having the barrier to iPhone development as low as, building a Web application, it wouldn’t make sense for Apple to only allow Mac using developers to build those applications.

Safari is a gateway to iPhone development.”

It may also explain why Apple chose to enforced their font rendering instead of using the native Windows sub-pixel approach ClearType.