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This pretty surprising. Given the industry standard with mining similar communications I'm surprised that Apple gave up a gigantic pool of user information like that.
Follow the money. Collecting user information is (in most cases) a distraction for Apple that could blow up in their face and lead to a serious PR fallout. See locationgate. See Path's address book shenanigans. It's in their best interest not only not to secretly collect user information, they also want to show that they care about stuff like that (because then they can point their finger at others). Apple cares abou…
Even further, iMessage accounts are tied to existing Apple IDs, which in most cases are tied to the user's iPhone and iTunes Store account. If Apple wanted to profile users of their iMessage system, they don't need to scan private communications, they'll just recommend albums, books and movies based on previous purchase history. It's more relevant to their business domain than trying to gleam whether or not Alice and Bob are in a relationship and expecting a child soon.