Allow restriction of editable dropdown field to picklist values only

When I create a Dropdown List field and allow it to be editable on the C360, it includes an additional option not available in the Dropdown List values: None.

 

I intentionally created this field so that it will always default to “NOT IN SCOPE” when new records are created, because I don’t ever want it to be null - it should always show one of two values:  “IN SCOPE” and “NOT IN SCOPE”

 

But there appears to be no way for me to force the field to only allow those two options when it’s manually edited.  I have to create a clunky work around for this by creating a rule (not a real-time rule, mind you, because RTR can’t update the Company object directly) that will look for any records where this field is NULL and set it accordingly. 

Product gaps, like what this appears to be, create additional administrative (and resource) overhead.