Now if this wasn't a visual novel there'd be no issue. A live-action film with the following description would be fine:
"A film that aims to raise social awareness of child and school related social issues through a story about four troubled heroines who, under unexpected circumstances, join forces with a young hapless teacher to fight against the evil forces that mean to exploit children."
Visual Novels (and to a lesser extent anime) just have a very seedy reputation in the west.
Does anyone actually believe this angle?
The premise sounds like its there to titillate a dark fantasy under the guise of a moral story. It definitely deserves a deeper discussion but I think Valve have every right to make the precautious move.