Web Form List view lacks adding row to child table feature

I have a Web Form that has a child table. The form is makeup of two fields only, a username and a child table to add rows of certain data.

When I enable the list view option in Web Form setup, I can read all properties, but I can not interact with the list. I have to add a whole new record.

Adding a record wouldn’t work, because the username field is the primary/ID/name field.

Is this a bug? Or just a limitation?