For iTunes, they do it because it makes them money by selling iPods and music. A better question to me would be "Why do they make Safari for Windows?". I'm not sure... is it to show different Apple software to Windows users and lure them into buying Macs? That sounds doubtful to me but I might be wrong.

But my question was a serious question: what do you think Apple has to gain from getting the Cocoa/Obj-C developers to write web-apps in Obj-J?