https://en.wikipedia.org/wiki/Macro_(computer_science)
See Definition of Macro.
A MultiBox software would be in a sense a Relay, as they only relay the Keypresses to the WoW Instances.
You press: "w" all WoW Instances register "w", and move Forward.
A Makro would be: you press "F6" and all Instances would register for example 3 seconds "W", then 2 seconds "S", or a Series of different Skill presses.
Whenever a GM talks to you you get a special GM Chat, thats not regular whispers.
Also, I didnt say: "I know they dont do that", rather that I doubt it. That means, I dont believe you unless you provide any sources stating the Difference. Because, whispering someone is not a very good Indicator if they do something illegal.
People could turn of their Chat, Ignore it etc, there is afaik no obligation to answer a GM in the ToS (again, if i´m wrong, provide me a source). And banning people because they dont answer a whisper would open a large can of worms.
No, they dont.
Answer: Blizzard doesnt see multi boxing as Violation, and Automation.
The MultiBoxing tools send each Command on all Instances of WoW, its allowed in that Scope and nothing more. You are not allowed to automate/Makro more than you can do with ingame Makros.
I dont know why that is so hard to understand, or what questions would be unanswered.
EDIT: To Clarify the GrayArea:
AutoHotkey in DiabloIII, I had a spec where you would press a skill every 3 seconds, after Research and a good amount of Googling, the only real Blue answer to that topic I could find was:
"Well, Technically its not allowed, but you probably wont get into trouble for that"
Thats a Gray area.