I heard here and there from the fandom that Sirius' behavior was too out of line. The misdeed would have been enough to have had him expelled. Not just cold-shouldered. And that he would have been begging for forgiveness to more to Severus instead of James or Remus. From what I see, people just looked over such actions and he was not even given a harsher punishment, which was akin to toleration of extreme bullying.