Well.. From a psychological point of view, it appears to me that Semper is a behaviorist. By that I mean that he believes that for someone to grow, the best way is 1-If possible, disallow "incorrect" behavior in any form, 2-if not possible, punish "incorrect" behavior (I use the term punish lightly. It could simply be making sure that "incorrect" behavior is automatically followed by bad consequences). And, of course, on the flip side, reward "correct" behavior.
Now, a few things strikes me as wrong with that. At the start, it assumes that the DM has, in part, a role of "teacher". It is his role to insure the player learns RL values from the game. I, for one, wouldn't want to play with such a DM. The role of the DM, in my view, is to 1-create the world, 2-advance, through various mean, plotlines, 3-roleplay all the NPCs. He is, however, a -part- of the story being unfolded, and so are the players. Yes, in the end his part might be "greater" in a way, but he isn't in a -real- position of authority over the players due to that. Yes, he is also the arbitrator, the one who decides which material is and isn't used, but I view that -simply- as a balancing role, not as a moralistic one.
Secondly, to espouse such a view means that every action that would be viewed as "bad" in the real world should carry a bad consequence, if not be outright banned. Therefore, any action from the players to "correct" any wrong without proper authority should be frowned upon. A village is attacked by bandits? Contact the authority, do not act as a vigilante. A rogue wants to pickpocket a rich, greedy and evil merchant? He -must- be caught, otherwise it is assumed the player of said rogue will become a modern Robin Hood, which is, after all, illegal.
But much more then that. Although behaviorism is still taught by some psychology teachers, and some books are still written espousing it, it has, by and large, been rejected as a valid psychological view for humans. It does work very well on dogs and very small children. But from 8-9 years old onward, the human psyche is much more complicated then that, and such a simple view do not suffice. It is my contention that the simple use of "evil content" in a RPG, no matter how vile, will not, by itself, corrupt the players to espouse such content in their RL. Especially evident, of course, if such material is used solely for the NPCs in a campaign. The only caveat -might- be teachings that such concepts even exist. Yes, if Joe Smith didn't even know that necrophilia -existed-, and he was to read about it in a book, the chances of him becoming a necrophile -might- be bigger then before he read about it. But even that idea is somewhat sketchy, and, let's face it, I doubt the BoVD will introduce new vileness many of us hadn't heard of before.
As for the idea that the very -existence- of a line of such books should turn parents from introducing children into the hobby, it reeks of "throwing the baby with the bathwater". After all, there is many books out there that I wouldn't want my children to read. What is the best way to prevent them from reading those books? To not teach them to read. Well.. Too late for that, Thank God.