C'mon Apple, isn't it about time you realize I might use another computer ?

So this is something that has bugged me for a long time about the iPhone and it still hasn't been solved. This title is not linked to my experience today, but the same error message I got would apply. I've updated my laptop to Windows 8 and since gone back to Windows 7. Obviously this included installing iTunes again each time. This doesn't usually matter, as I manage my music through the cloud most of the time, but today, when I wanted to put a track on my iPhone that wasn't bought from the iTunes store, I get this :

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via i50.tinypic.com

This is a pretty common issue. When I tried to do the same for another iPhone user some time ago, he had a new laptop and wanted to put some music on his iPhone which had been synced with his old one, but would it allow it, NO !

This isn't the case with an iPod Touch or iPad for example, which confuses me, because as far as I know, they all run iOS. Why is Apple doing this ? It's possibly the single most annoying thing about owning an iPhone and will seriously make me think about switching to another platform, because I don't want to be on tender-hooks everytime I change my computer.