The iPhone apparently adds "Sent from my iPhone" to every email, which, although it's straight from Apple, can be mistaken for bragging by the user instead. And since it's easier to leave it on than turn it off, and you don't see it when you're sending messages, you'd probably not turn it off, at least initially. Now imagine if, when setting up email on your iPhone, it asked you "Would you like to advertise for Apple with every email you send?" You'd probably click "No", which pretty much means this "feature" should not exist at all.
Mokalus of Borg
PS - I seem to have been writing a bit about the iPhone lately.
PPS - Probably means I subconsciously want one.