The distinction here is that the iPod/iPhone is the product, while iMessage is merely a feature. By creating iTunes for Windows they were expanding the potent

The distinction here is that the iPod/iPhone is the product, while iMessage is merely a feature. By creating iTunes for Windows they were expanding the potential customer base for iPods. By supporting iMessage on Android they would not be expanding the potential customer base for iPhone. If anything they would be shrinking it if they think iMessage is a compelling enough feature for people to switch from Android to iMessage.