Header Presentation with "String" Kind isn't edita

I've found that for a header presentation with Kind set to "String" the presentation isn't editable. If I change the presentation kind to "Small HTML" or "Text" the presentation becomes editable. Can someone explain why a "String" presentation in a header isn't editable? Is this working as designed? Or is it a bug?
Thanks,
Geoff Alexander
Thanks,
Geoff Alexander