[§5Aki]

Mainly I'd say it's a rule because people like to use it for disrespect. If this wasn't a rule then the vc would be constantly filled with people yelling at each other to shut up, this and that, etc.
As Fish has said it's also mainly towards females and or people with high pitched voices (such as children who will constantly chatter because their kids). As someone falling into the category of female, I've been told to shut up numerous amounts of times. It's just not okay.
However, if it's, as Star said, used in a more playful manner with friends just poking fun then, yes, that's okay in my book. I'll be lenient as long as they don't start up with constantly saying it to each other (which would then make it mic spam) or starting to sound disrespectful.