In the essay Fact and Value, Peikoff says:
Yet, in his "DIM Hypothesis" book he urges us to read up the works of the Big Three philosophers, Plato, Aristotle, and Kant. Is this a contradiction?
Is it moral for an Objectivist to read the books that criticize Objectivism ? Am I a fake Objectivist, if in my bookcase I have "Critique of Pure Reason" and/or I even recommend to others to read it as well. What about "The Passion of Ayn Rand" by Barbara Branden? What about Marx "Capital" or Hitler's "Mein Kampf" ? The Bible?
What about extracting interesting points from non-objectivist material, for contemplation? Consequently, what about communicating on forums and websites such as "Atlas Society" or "Objectivist Living" and/or attending their seminars to learn what they have to say (which may be interesting and valid in part)?
What about being a member of the local philosophy club, and study together topics from different schools of philosophy?
It's not clear why the questioner would think this is contradictory. If one is to legitimately reach a conclusion that something is false and wicked, then one needs to study it to do so. And if one has already done this and wishes for others to reach the same understanding, as Peikoff does, then of course counseling them to study the matter so they may likewise see is in order.
So then what about the morality of reading books that criticize Objectivism? The essence of immorality, for Objectivists, is the opposite of reason: actively closing ones eyes to facts rather than pursuing the truth. It would be patently irrational and therefore immoral to avoid a criticism merely because it is a criticism.
And what about studying the thoughts of non-Objectivsts? That's nearly all of the thinkers in history! Look, it's this simple: Objectivism is not coequal with all truth, nor even with all philosophical truth. Not anywhere close. So if you're not studying thinkers other than Objectivists, then as the kids say, "you're doing it all wrong."
Whether people should spend their precious time studying things they know to be dishonest or otherwise without merit depends on their context and hierarchy of values (very often it would be a sacrifice and therefore wrong).
Finally, studying and thinking about something doesn't require giving active support to it (granting it the immense value of one's "sanction"). So whether one should hang out in this or that forum or seminar or whatever is a matter of being careful to support one's values rather than the opposite. Studying Mein Kampf and talking with Nazis is one thing -- joining the Nazi party is another. Studying the Bible and talking with Christians is one thing -- joining a church is another. Etc.