I've wondered about this myself, since I've written a simplified transition handler for use (only) with 1.0 transitions. It's part of this package:
https://github.com/watusi/jquery.mobile.simultaneous-transitions(The focus is set in the transition handler).
I've tried to streamline the transition handler as much as possible. And, so, I've wondered how necessary settings the focus is. I've experimented with commenting-out the line that sets the focus, to no apparent ill-effect.
It's not clear to me why it should set focus.
Shouldn't this at least be an option that can be set?