A Boring Dystopia

That's not exactly correct. So first, in order to recieve SNAP payments as a seller, you have to be authorized. This provides limits and oversight on government money, making sure it is going to certified retailers, or farmers markets, not just anyone. That means that everyone who is authorized must meet and continue to uphold certain requirements of the law or else lose their authorization. One such requirement is the equal treatment provision which prevents that seller from treating SNAP buyers any differently, whether positively or negatively.

So, A) there is not an actual law preventing anyone from giving a SNAP recipient a special discount. 1) If I were an authorized seller, I might lose that status and no longer be allowed to take SNAP payments. But I would only have broken a requirement for that privilege, not the law. 2) If I am not a SNAP authorized seller, there are zero reasons I couldn't give out discounts to SNAP recipients. There would be no consequences at all.

B) The reason I can't charge SNAP recipients more should be obvious, becuase it would mean the government is getting charged more for their beneficiaries to receive less. But it might seem counter-intuitive that they can't then charge less to them. I think there are two (threeish) major reasons for that. 1) part of the reason that the current system works, where SNAP recipients have a preloaded card that they charge, rather than stamps or other currency used to be used, is that it maintains some level of anonymity so that even that cashier doesn't necessarily know that anything is different between their transaction and the next guy's in line. 1.5) This anonymity and the lack of special treatment for the entire process means that people who need it are more likely to participate in the program because they can maintain dignity and not be embarrassed for their need, as plenty would be. 2) People are already attacking SNAP recipients for getting a handout. Can you imagine how much anger SNAP targetted discounts would make those people? To protect a much needed program, they want to make sure that there is as little reason as possible to attack it. So treat them as close to equal as humanly possible.

Also, there are allowances for exceptions to this rule already, such as discounts on fresh produce and whole foods to promote healthier purchases with SNAP dollars.

EDIT: Forgot to address your question about why they couldn't pass a law to require the same price and discount be given to everyone. They probably could. I don't think there is any good constitutional argument to say they couldn't. But no such law currently exists, which is why it is not a legal problem at this time.