Wouldn't the simple answer not be: because XML is not TeX?
I see your point for tilde: with finalizers in mind I was already in the stomach, while mouth was looking at the menu. Teaser for S01E02: finalizers.
I still would expect unicode nbsp to be expandable, otherwise I would have to treat it somehow (no problem with that). Remember times when non expandable/shrinkable nbsp was the first clue the book was typeset in Word? I've checked it now and it's still the case.
Thank you,
Jano