Hmmm...can one imagine that New York Times op-ed if a bank said they would stop processing transactions for some other perfectly legal but perhaps morally objectionable activities like say abortion clinics?
I don't think you can make the public-interest case against abortion: a very small number of people would argue that mass shootings are a good thing, but it is not clear that a large proportion of people (in the UK at least, the US is a bit of a mystery to me) would argue that the act of abortion does any damage (or at least, nothing like killing someone).
1) leverage / scale (a person with a gun can kill more people in a shorter space of time) 2) it is contended whether or not this is killing someone, whereas noone (probably, almost) contends whether a school shooting is killing anyone
Even with large numbers of people thinking abortion is murder, this is overwhelmed by the number of people thinking that shooting someone is murder. Since murder is considered (probably) by a similarly large majority, and a gun can create more murder than an abortion, you could make a far stronger public interest case for CC companies to apply pressure to one rather than the other.
The main point is that while almost everyone will agree that murder of humans is undesirable, not everyone will agree that abortion is murder (_even_ in the United States), which means that it is not categorically clear that the outcome of abortion is a negative one, while it is clear that the outcome of a shooting is a negative one, and therefore there is a stronger public interest case in applying commercial pressure to sales.