What's the strategy here? What's in this for Apple? Why not Facebook?
Apple has previously tried to do a deal with Facebook to presumably implement similar features - those negotiations weren't fruitful, perhaps a stroke of good luck for both Twitter (#2 in the space) and Apple (Twitter less likely to be a competitive threat to Apple).
In some respects this is dangerous for Twitter too. If Apple users get used to these features and use them appreciably, Apple may move to re-implement on their own platform a la iMessage (a replacement for carrier SMS). The smart guys at Twitter probably have a pretty good understanding of how this commodifies some of their platform and how to manage that over the long term.
/r
As for what Apple stands to gain? I don't know for sure but it would be a good experiment for them to see how many people would use a social feature like twitter when integrated and how people socialize and leverage that data both for learning how to improve their own social elements as well as maybe strike a monetary deal later. Who knows. It's too difficult to speculate. For all we know, its just good UX that could draw in more love for Apple products.