That's an interesting point. Have you seen such a situation? Have you seen a situation in which the opening inverted question mark or exlamation point is in regular typeface followed by text in a special typeface?
I think the purpose of this rule is to establish that the opening inverted puncutation, which is unique to Spanish, assumes the typeface of the text it is associated with in the same manner that the ending punctuation does. In English, opening punctuation, such as quotation marks and parentheses are not included in the typeface indicators regardless of how they are printed. The same is true in foreign language. This rule covers the unique instance of the inverted question mark and exclamation point. I think most print situations are covered here.
It you do have such a situation in print, where the opening punctuation is clearly not emphasized, I'd like to see it so I can discuss it with other. I would be reluctant to separate the opending puncutation from the text it belongs to with a typeface indicator, but as I said, I would discuss it with others if you have such a situation.