Web and app developer. Coffee, beer, and running, in any order.
In hindsight, it makes sense. The admin class is how Django knows how to render the form for the model.
Additionally, using autocomplete seems like a nice touch for that field (which requires an Admin registered for the foreign key model and is how I stumbled across this).
I thought I was having issues because I was using a through model on a many-to-many relationship, but nope.
Ok, so in Django if you want to use a TabularInline or StackedInline that contains a foreign key, and want to be able to create new records for that foreign key from the Inline, you have to define and register an admin for that foreign key model.