thucydides interpretive answer

Why do the Athenians make it clear from the beginning that they do not want to speak of justice?

I think that the Athenians were reluctant to speak of justice to the Melians because they were aware, on a conscious or sub-conscious level, that what they were commanding the Melians to do was not just. Every human being, whether they are considered good or bad, has a conscience, an inner knowledge of what is good and, likewise, what is bad. Even if the Athenians were not consciously aware that what they were doing was wrong, their consciences were. Because of this, the Athenians were very uncomfortable discussing justice with the Melians, and made it well known.
I also think that the Athenians might have sensed that the Melians were more inclined to justice than they were. They felt that if they started speaking of justice, the Melians would have the advantage. All of these feelings, I feel, contributed greatly to the Athenian’s avoidance of the topic of justice.