Not sure why everyone is so surprised. This goes hand in hand with apples goals in creating a seamless ecosystem for the customer. It's part of their brand, mission and identity to have this tight/strict policy. It would actually hurt their brand if they derailed from this.

Apple is an end to end ecosystem. It has a straight line vision. I think they would be wrong to not do this.

And this is coming from a privacy / open source geek. I run my own mail servers.