Add field to only ONE term!

On /admin/content/taxonomy_fields/add_existing and /admin/content/taxonomy_fields/add_new it says "Select vocabulary ..." and I see a number of comboboxes with the terms from various vocabularies.

As far as I can see, I cannot actually select ONE vocabulary, but I have to select a term from each vocabulary (or leave it at the default top choice), and the field will be added to the one selected term of each vocabulary. Then I have to remove the field from those terms that I didn't really want to select.

/admin/content/taxonomy_fields/manage_terms is even more confusing, because this will apparently set Display ancestor-fields for one term from each vocabulary at the same time.

I think the comboboxes should have a <none> choice, so that I can select one term from only one vocabulary (and leave the other vocabularies at <none>). Am I missing something?