I don’t think that concern for injustice is something universal or even necessarily widespread among autistic people. That is, I believe that there are plenty of autistic people who do not give a fuck, eschew social causes completely, feel misanthropic, etc. Rather, I think that the examples you encountered represent that segment of autistic people who do care, and due to the difficulty that autistic people have with understanding social roles and social boundaries, they felt a need to act and did so, when normies might not have felt a need or would have held back.
I think it's more accurate to say that autistic people tend to express their emotions much more intensly than non-autistic people, when they do decide to express them. Most non-autistic people will tend to consider social norms because they innately understand to not disrupt them. Autistic people, especially when they've decided on something, are completely willing to eschew social norms to right a wrong or express their stance or something similar.
That's not to say they're more or less empathic than others, that's just as variable as with non-autistic people (basically just because you're autistic doesn't mean you're incapable of being an asshole). However because autism partially includes a stunted social growth, the decision making process that might prevent a normal person from say, commenting on someone being rude in public (ie. Not causing a fuss, not wanting the attention to be focused on them) just... isn't quite there. That can both be good and bad depending on the situation.
At least that's been my experience having been a cashier (a couple years ago now) and having had customers who were really obviously autistic. They tended to lean towards being really expressive/opinionated, just... not especially caring if expressing that opinion is appropriate. That can be amazing if you're dealing with an especially frustrating Karen who tries to haggle prices in a fucking supermarket because she didn't bring enough cash (no cashier wants to risk putting their foot down and that blowing up in their face but in supermarkets you do not haggle the total, it's just not a thing), so having another customer back you up feels very solid/helps when management inevitably has to be called. And it can equally be frustrating when they start counting out the VAT that the cashiers computer calculates and they decide the rounding isn't to their liking/when they figure out there's a discount on a product and it isn't applied and then they try to get the cashier to correct it instead of customer service.
I don’t quite see how pointing out to the cashier that you’re being overcharged makes someone neurodivergent. I catch wrong prices all the time and say in a friendly tone “hey can I just get you to double check that, I though I saw it was $x.” If they then say to take it up with customer service thats fine, I don’t press it, but pointing it out as soon as you notice is the best response and in many smaller stores there is no one else to deal with.
difficulty that autistic people have with understanding social roles and social boundaries
In many cases those are understood just fine, but with the caveat that they're not optimal for every situation. There are times when it's appropriate to ignore those criteria (eg to prevent an injury or mitigate some immediate risk), and the people wringing their hands about what to do are unhelpful or even in the way.
More than once I've seen bystanders weirded out by an unpleasant situation stand there repeating 'someone should call 911' over and over without being able to complete the thought and do it themselves. I have enormous respect for the people who are sufficiently present to evaluate and situation and act in the moment, even if their actions are not always perfect; I have negative respect for the people who revert to infantile paralysis or try to dissuade others from acting int he absence of a recognizable authority figure.
By the same mechanism, Autistic people may be less likely to agree with the social group's view of an example of injustice. Especially if the wider agreement has a contagious element that oversees its facts. Instead, ASD people may be more prone to attending to injustices that they can individually percieve.
It wasn't about "autism", but there was a study done a while ago, where it was the "cranky independent" people who intervened most. They might have been left-styled or right-styled; what they had in common was that they didn't go along with the crowd.
Every autistic person I've met (and I've met a LOT of them) has this similar drive to stop injustice when they see it (regardless of how much they hate society in general).
There are plenty of people on the spectrum here who will disagree. Moreover, as someone in that category in Eastern Europe with local peers in that category, I find that quite a few of us share the fatalism that the region is known for and can be suspicious or mocking of attempts to stop injustice. Indeed, could it be that in your region (or in the region of the linked study) concern for social justice is simply a common feature of society such that autistic people will share it, and therefore this is a matter of culture and not autism worldwide?
As a fellow amateur batman fantacist and definite sperglord I think it's not so much about justice necessarily, as much as it is about the system. I need for the world to work on a rational systemic way to be comfortable and counter examples are deeply unsettling to me.
