Why did Snape spare Harry, Ron and Hermione for attacking him in Prisoner of Azkaban?

Question

During the eventful night at the end of the Prisoner of Azkaban, Harry, Ron and Hermione simultaneously cast the Disarming spell at Snape from the Shrieking Shack, sending him flying back and knocking him out.

Hermione rightly observes: “We attacked a teacher. . . . We attacked a teacher . . . ,” and that “Oh, we’re going to be in so much trouble —”

However, events so transpire that Snape was to arrive the triumphant at the school holding the others (except Lupin) bound. But he then tells the Minister who arrives for Sirius that the trio had been Confunded by Sirius, effectively saving them of the charge. This could be seen as a concession in the light of his sense of double triumph, of having done a good job of cornering a most wanted criminal, meriting the Order of Merlin, and of having his 'sweet vengeance' on his arch-nemesis Sirius by showing him his way to peril

But hereafter events transpire in even more unexpected and strange ways that Harry and Hermione manage to let Sirius escape (almost about the same time) leaving no clue as to anything. We know that this caused a severe disappointment for Snape. When Dumbledore also managed to acquit Lupin, he must have been beside himself, for as Lupin said,

"That was the final straw for Severus. I think the loss of the Order of Merlin hit him hard. So he — er — accidentally let slip that I am a werewolf this morning at breakfast."

Now the question is why didn't he implicate the trio now on the charge of attacking him at the Shrieking Shack? That was all he could do now, and Dumbledore could not have easily interfered here, as their wands would have told the truth, via Priori Incantatem. And the Confundment theory was propounded by himself, and could as easily be withdrawn at will.

That Snape recognises all possibilities is beyond doubt as he had said as early as from the Shrieking Shack, “Miss Granger, you are already facing suspension from this school."

So, why did Snape not go on to get them punished, even expelled?

He tells Bellatrix in Half Blood Prince (at Spinner's End) that "I have done my utmost to have him thrown out of Hogwarts, where I believe he scarcely belongs", but the question is, was he telling Bellatrix the truth, or did he really want Harry to be protected inside Hogwarts, besides sincerely wanting him alive?

Answer 1

Snape is deeply conflicted about his feelings towards the son of his school days chief tormenter on one side and the unrequited love of his life on he other. He wants to keep Harry safe for Lily's memory but sees a lot of James in him at the same time. She was the reason he left Voldemort. He knows Harry will be safest in Hogwarts because he will be aware of Dumbledores concerns that Voldemort could still be out there. His threats and attitude toward Harry is an instantly emotional reaction which is tempered over time and I think deep down he would not follow through and actually see Harry put in danger. He also is perhaps going to feel pretty foolish at being overpowered by students so there is possibly a lot of pride going on there as well.

Answer 2

By the time Snape got hold of them, they were already injured, unconscious, almost attacked by dementors and very vulnerable in the Forbidden Forest. So he took them to the hospital wing. Moreover, by the time Harry and others gained consciousness in the hospital wing, they were already surrounded by other teachers and the Minister, who were thinking Snape was the one who saved them. So it's also a matter of personal reputation for him, which was why he didn't punish them outright. He would rather appear as a hero in front of the Minister for fighting back a werewolf and apprehending a dangerous loose criminal, than punish children when they were so weak and injured. He also had an explanation for their behavior by this time, that they had been confunded while in the Shrieking Shack (so they seemed to think Peter Pettigrew is alive and Sirius Black is innocent).

Answer 3

In his attempt to defame Sirius and making the case against him even more concrete, Snape had also said this to Minister of Magic that the trio had actually started believing the convict murderer and as per Snape's words, the trio were under Imperious curse casted on them by Sirius. This conversation happened when Ministry had recaptured Sirius and the trio was lying in the Hospital Wing.

Snape knew perfectly well that he can play only one card. Either blame the trio for attacking him or blame Sirius by telling the minister that he had casted an unforgivbale curse on the trio. Snape thought the latter was a better option and will ensure Dementer's Kiss to Sirius and opted for it. Little did he knew that Harry would go back in time to release Sirius :)

Answer 4

If Harry were expelled, he would have no safety from Voldemort except for at the protected home at 4 Privet Drive, and he would've gone against everything he had been doing for the past 13 years in serving Dumbledore.

Answer 5

I think it is much simpler than other answers suggest: Snape didn't report being attacked by the trio because he simply didn't remember the attack. It is well-known, after all, that people who receive serious blows to the head - which Snape must have received, having been slammed into the wall (or was it the bed headboard?) - often result in significant short-term memory loss, and so people often do not remember how they were injured.