Roland is right, but I wanted to give some additional explanation as to what constitutes a "morally justified" thread.
"Alignment" is a feature of a game system with a reasonably strict set of guidelines; "moral justification" is a feature of the real world with no such clarity. If your discussion of a character's alignment veers so far afield from the game definitions that only real-world judgments and examples are being used, then you're not discussing alignment anymore, you're discussing morality. And in-depth discussions of personal real-world morality almost always tread into the no-politics/no-religion end of the pool.
Likewise, if everyone on your thread agrees that a character would have a listed alignment of Evil before a ten-page debate begins on his actions, then you are not discussing his alignment anymore. If you are attacking or defending a fictional character for being who they are, you are engaging in a "morally justified" debate, not a discussion of their alignment.
The locked thread regarding Tarquin began from the premise that Tarquin was Evil and then went on to discuss whether or not his evil actions were acceptable. It sought to argue that Tarquin shouldn't be admired by readers because he had committed evil acts, which is essentially one poster telling other posters that they should follow his own personal moral compass. It then followed with various posters defending Tarquin's actions, often through real-world benchmarks, even while acknowledging that he had "Evil" written on his character sheet. Thus, it was not substantially a debate about his alignment, but about whether having such an alignment was a positive or negative thing in either a character or a leader. In other words, whether being Lawful Evil was morally justified.
Hopefully, that makes things a bit clearer. As far as announcing it as a specific rule, we mostly see it as falling under the heading of no-politics/no-religion, or, in some cases, of telling other posters what to do (in this case, what to believe), both of which are already established rules. However, maybe we'll tweak the wording in the Rules of Posting to make it more clear. As Roland said, though, we are not currently infracting people for straying into this territory on account of it being a borderline case. If we enshrine it in the rules, we will begin doing so.