All right, hear me out--I am not saying that it's BETTER if Sprout weren't to be punished, just that it's completely fine if he ISN'T. Furthermore, if a person in real life did the same things Sprout did, of course they ought to be punished. I am keeping in mind that this is a fictional world from a childrens' program.

First off, let's ask ourselves: What is the purpose of punishment? There are two main reasons: Justice and discipline. In these cases, justice is not only to inflict payback against an offender so that they fairly receive as much suffering as they caused, but also to give closure and a sense of contentment to the people who were hurt. Justice has to be DESIRED. Discipline is to give someone an unpleasant feeling after they have committed unwanted behavior, so that they are less likely to re-offend in their efforts to not experience the unpleasant feeling again. Discipline is needed if someone needs to be taught a LESSON.

For example, let's look at Starlight Glimmer. She wasn't really punished for her crimes. Why? There was no need for justice because Twilight didn't WANT to prosecute her, and there was no need for discipline because Starlight Glimmer had ALREADY learned her lesson.

That can very well apply to Sprout too.

Sprout does not need to be taught a lesson because he already KNOWS better now. The entire reason he destroyed Sunny's house was because he was certain unicorns and pegasi would be deadly if magic came back. But now that he knows they're not a threat, he's not going to repeat his destructive behavior. Sure, there was unfortunately that moment at the end where Sprout makes one last desperate attempt to seek his mother's approval, but look how remorseful he is after he snapped out of his power-mad, fear-driven rampage.

There were some scrapped storyboards in which Sprout tries to run away, and then Izzy grabs him with her magic and he gets forced to apologize while still being terrified that Izzy could fry his brain. A lot of people prefer that ending idea, and I used to as well, but I realized that the current ending is much better for Sprout as a character. In the scrapped ending, he tried to RUN AWAY from his problems. In the finalized ending, he didn't run from the reality of his actions and bravely faced them no matter how terrible he felt--he even walked up next to Queen Haven, a pegasus. In the scrapped ending, he was FORCED TO APOLOGIZE even though he didn't yet understand that the other pony breeds aren't a threat! He still thought the unicorns could fry pony brains, and would probably go on being utterly confused that everyone wanted him to apologize for trying to stop something they ALL used to fear. In the current ending, he doesn't SAY sorry but to be fair, nopony else ever outwardly apologizes for formerly being racist. And you can tell how regretful he is just by watching him; he has his ears lowered moreso than just about any other pony ever did in the movie.

So anyway, with the finalized ending in mind, punishing Sprout WON'T prevent any future crimes committed by him, because he WON'T commit such big crimes anymore anyway, now that he realizes the truth. Things will be the same whether or not he is disciplined.

So what about the reason of justice? Should he still be punished anyway so that Sunny can get justice for having her home destroyed?
Well... the big question is, does she WANT him to be punished?

Everypony seemed to accept and forgive each other in the end despite all their histories of hating each other. Sunny said love and friendship are the true magic and that EVERYPONY needs to come together. Everypony has a past they're probably ashamed of now; Sprout just took things farther than the others. Queen Haven and her daughters are seemingly forgiven for lying about being able to fly, despite how angry all the pegasi were before. Phyllis is seemingly forgiven despite the weapons she made and the propaganda she spread. The ponies want to leave everything behind them and are too delighted about having their magic restored to bear any grudges. So, unless ponies WANT Sprout to be punished, he won't be. And it doesn't seem like anypony would.

Plus he's already somewhat faced repercussions via the shock of almost running over his beloved mom--that must have felt TERRIBLE. I have the Warriors book series in mind, in which Graystripe has kittens with a cat from a different Clan--which is forbidden. However, his mate passes away during birth. Despite breaking the code, Graystripe doesn't get a proper punishment, because "the pain of losing his mate was punishment enough". I think the guilt of realizing his actions weren't necessary, and the horror of almost hurting his mom, are enough of a repercussion for Sprout.

Besides, if we punish Sprout, then pretty much every single other pony in Equestria would need to be punished to some extent for harboring prejudice in the first place.

You may say "That's not how things work! People can't get off scot-free just because they look sorry and other people don't ask for them to get punished!"

I repeat, I'm looking at this from the context of the My Little Pony universe rather than real-world rules. And in case you need a reminder, here's a list of SOME of the bad actions that MLP:FiM characters committed that went unpunished:

• Fluttershy selfishly kept one of the Parasprites in secret, despite knowing how harmful they were. Because of her, the Parasprites multiplied again after the Mane 6 efforts to eradicate them, and ended up DESTROYING much of PonyVille.
• Spike let his greed get the best of him and conducted all sorts of theft and property destruction in Secret of my Excess.
• Starlight Glimmer ruled over a town and brainwashed ponies into abandoning their identities for a much longer time period than Sprout was emperor.
• Rainbow Dash abused her power to use the weather factory for her own purposes in Tanks for the Memories, causing a lot of damage and destruction in the process.

That's just a few.

Punishing Sprout won't change anything. He's obviously someone who hasn't experienced kindness or been listened to throughout his whole life, and being jailed or whatever will only isolate him further. He needs to feel respected, trusted, and important to others--and the only way he will become a better pony is not by being punished, but experiencing support and companionship from more ponies besides his mom. And at the end, his mom's reaction of completely dismissing him and walking away instead of addressing the matter definitely isn't a way of helping.