That setting is just to enable the categories to be selectable in the form. The user is still required to select the category on the form. To see what I mean, try checking a few more categories in the settings and then looking at the form. You will see that the form’s category field always includes the same categories that are selected in the settings.
Oh.. but I set “Post Category” to be hidden. Posts should end up in the category I selected (since users can’t select anything) 🙂
Ah, then you need to scroll down the settings a bit further.. look for the following settings:
– Use a hidden field for submitted category?
– Category ID for hidden field
These are the settings that apply if you are not showing the Post Category field.