I would say namely because the parent record is in the record buffer and subject to being edited.
Have you tried implementing a security scheme that would leave the parent record in read-only mode on the window where the editing is occuring?
It is a good question. It is definitely a nasty 'gotcha' that has bitten me several times.
Maybe because this method makes it possible to program transactions in FileMaker?
...Aha! The answer has just occured to me ... the reason is an undocumented fact of filemaker relationships:
A create-relationship from an empty field to a field with an id/auto-enter, fills the empty field in the source table!
Strange but true .... check out the example file.
This method of record creation has been around forever and has been documented forever.
It does nothing to solve the problem at hand.
This is one reason the session model is used; the "top" parent record is an individual record unique to the user.