Maybe this is just a fundamental moral divide but a lot of people do think it's reasonable for someone to say that a grocery store shouldn't sell guns, or that a payment processor shouldn't process gun sales. Just for one example.

If you love guns, substitute something else you dislike. Currently people are aggressively pushing to prohibit the sale of adult entertainment, for example, and doing so by trying to get payment processors to block those transactions.

It's very common for people to hold the opinion that stores and payment processors should be opinionated as well and discriminate in terms of what they allow to be bought or sold.

And that's completely ignoring the question of how laws governing commerce interact with this. If you do business with a sanctioned entity the government's going to get angry real quick AFAIK, regardless of whether the business was itself harmless.