I haven't seen it mentioned, but there really is no overarching "logic" behind the forum rule regarding this.
My understanding is that it stems directly from the founders of the website, and their respective religious affiliations.
Thus we can't say U or M words, but we can refer to Roman Catholics as heretical/schismatic papists all we want. I'm not challenging or complaining about the rules, just pointing out that they don't appear to be based on any consistent application of theological, ecclesiastic, or socio-religious understanding. Them are just the rules.