Hi Jason,
This is because by setting the association as 1-1, you are essentially setting both entities as the owner of the association (see One-to-One Association):
"A one-to-one association is created by setting the owner property to Both
"
No such thing is allowed between a NPE and a PE, because that would mean that the association could start from the PE, and when the NPE entity "dissappears", the PE will no longer know where to find it.
I hope that solved your question, best regards!