Shock value: the potential to provoke a reaction of sharp disgust, shock, anger, or fear.
When people say, "That <Insert Act of Violence> was only added in for shock value." they are not complimenting an author.
But if they say that something was added in for comedy value ... not so much. And how about, "You only added that in for interest value," or "Come on, you only wrote that bit to make me feel a sense of wonder and awe." I can't remember ever seeing that.
So what is wrong with shock value?
So is the accusation of shock value just made by those upset at being more shocked than they wanted to be? Sometimes it might be.
I think the truth though is that "shock value" is, or should be, aimed not at criticizing the notion that a book might shock us but at clumsy implementation. It's hard to imagine that many people really think a book should never shock its reader ... but doing so by having a ghost jump out and say "Boo!" or by some other method that a particular reader does not find effective may provoke calls of "shock value".
TL:DR - generally it's not the shock they're complaining about, it's seeing the mechanics of the process.