I'm not an expert on tennis etiquette, but I think Sharapova uses it to intimidate her opponent, and yes that is outside the rules. I agree WP it is a form of cheating that should be discouraged strongly at the junior level...
Banning seems extremely harsh for a 9yo girl though, sometimes I think we interfere too much in society, we're becoming a bunch of Mrs Mangles that want to intrude deep into people's lives via the Herald Scum.
The other side of me thinks ban the cheeky little copycat until she learns her darn well lesson.