I don’t know if it’s lack of imagination, flawed logic, or organizational momentum, but a lot of organizations have ended up with a structure that puts email, social, postal mail, and even telemarketing into separate silos. Sometimes they are silos within a single department; in more than a few cases the silos span two or three departments in a larger organization.
As you may have experienced, org structure based on channel is a recipe for internal conflict, disjointed communications, and a whole heck of a lot of bad meetings. There’s no good argument for it.
Structuring by audience is the only approach that makes sense. That way, there is a natural flow toward integrated communications and there are clear lines of responsibility in terms of who is ultimately responsible for stewarding each distinct audience.