Hi Charley
You won't like my answer, I can pretty much guarantee.
So, is this a bug?
No.  The software is behaving as it was designed to (before we got involved).
 If it's not a bug, can someone explain the logic
behind having purchase orders behave this way?
Probably Dieter can.  Taking a guess, I would think that this was
designed for audit control purposes.  However, you are right and this
is incredibly sub-optimal.  We are looking at re-designing this for
1.4.  Now would be a good time to get feedback on how you would want
it to work.