Because there's one thing the NSA isn't tapping: the internet.
It's implausible that they've decided to capture text-messages, but turned a blind-eye to iMessage.
But they may not be able to crack iMessage.
I like your world--let's live there instead.
I think it has got to the point, all revelations considered, where we can't hope or assume any encryption is safe.
Technology can't prevent this, we must change laws. Which at present, seems almost as unlikely.
Technology can help to a certain extent, but only if open source and handled client side. No amount of encryption on Apple's (or any company's) end will matter, because users can no longer trust them.