COW pages and FL_PUNCH_HOLE

I don't see a discussion of this anywhere ... and I thought linux-api was
the right place to ask.

I see holepunch as being "like ftruncate() but for the middle of the file".
So it feels weird to me that MAP_PRIVATE behaves differently; truncate
will rip out COWed pages, but holepunch won't.  Is there a good reason for
this behaviour, and is it too late to change?