This is a good point, but there is a difference between criticism and shutting out. It is likely the latter that Charlie was most concerned about — perhaps a prior bad experience led him to institute the rule — and furthermore it's unclear to me that simply expressing disagreement politely is enough to be considered criticism.

Additionally, simply as a matter of effectiveness, you are not likely to change any minds by dismissing ideas out of hand or even presenting an eloquent and sound rebuttal. A person who seriously entertains idea of the paranormal has so many holes in their faculties for determining truth that you really have to start from the very basics. And even then it is not worth the time or effort unless you are very close to this person.