The reason for this is that by choosing one letter per tribute (rather than one letter per gift), the user is choosing a type of record that doesn't have soft credits. Gifts have soft credits, while tributes do not. 

Since the soft credit is attached to the GIFT, rather than the TRIBUTE, the soft credit option on the General tab of Honor/Memorial letters only comes into play if the option on the fields to include tab is chosen for one letter per Gift.