The only thing I can figure is that personal information was being posted, like either RL info, or passwords to mod accounts or something. If they got banned, but the info could still be accessed, that would definitely be a problem, BUT there's a better solution than just making posts inaccessible:
Add the ability to delete a post, but make it so that if a mod deletes a post, it only becomes "suspended", in that no one can read it at all, even if they try to quote the blank post, but it requires the approval of either ioi, or 2 other mods to approve the suspension to make it a permanent deletion.