Most companies have massive T&Cs which, if we ever read, we'd have objections to. As Stripe is a middleman in payment I can see why they want to protect themselves from any transactions where there's a higher than acceptable risk of the seller breaking the law.
In the US a business needs to have a license to sell firearms and that business has to follow lots of laws, and Stripe probably doesn't want to be in the business of policing their clients.
You also can't sell things like mortgage advice and e-cigarettes, even anything that's considered "flammable", so it seems a quite a jump to call this "anti-rights" rather than simple legal protectionism.
The problem is that there are a lot of laws around gun purchase, especially globally, and the legal repercussions (federal and civil) of an illegal gun sale can be a LOT more than the repercussions of selling illegal beer.
That is why 'high risk' processors charge a considerably higher % and per transaction fee and generally hold a percentage (I've personally seen as high as 50%) of every transaction for 1 month or longer (I saw an extreme of one holding 50%, releasing 25% after one month and the other 25% after 3 months before releasing it to the business as you generally have 60 days to dispute a charge).
That’s why I said I hoped you were trolling, because the alternative is that you have a strong opinion about something which you have thought insufficiently about.