Usual category applies to all, not only email

I’ve been using the Write by Email system for some time now and noticed that setting the option ‘Usual category’ not only applies to Writing by Email. It applies to regular entries as well. Still, the option is displayed within the ‘Writing by Email’ section (admin/options-writing.php). There should be a difference between the blog-by-mail category and the category where the regular entries reside.