I've never really gotten how it's generally allowed in matches to be honest.
What is wrong with it? It is almost same as well disguised dropshot, sly, unexpected, totally legit. If you think tennis is only about hitting 200km/ h serves and 150 km /h forehands... well it is much more. You cant ban the shot, because it is too weak and short, this is ridicilous idea.They even dont ban hitting your opponent with 150 km /h flying ball( = bullet ), why should they ban totally harmless slow shot ?