A: It’s smart.
One way to think about what’s happening today is to compare todays model of mobile platform (apple, android) and the user generated content (fb, twitter, YouTube, instagram) to the tv networks and cable companies. The mobile platform vendors like apple will spend a lot to avoid being a dumb pipe.
Also, in network effect businesses, slight differences get magnified hugely as scale increases. If twitter works 10% better on iOS and people want twitter, it becomes a huge competitive advantage.