Let's review some basic Business Ethics 101.
The big reason why businesses don't want to sponsor or support political extremism in general is because that will most likely tarnish their brand images.
Take Coca-Cola as an example. When you think of Coca-Cola what are the first things that comes to mind? Chances are the iconic soda brand reminds you of Santa Claus, polar bears, barbecues, road trips, having a good time, and America; a clean and wholesome image that lead to the huge success for the Cola-Cola company.
Now what would happen if say tomorrow Coca-Cola started sponsoring the Ku Klux Klan, they donate money to various neo-Nazi organizations, and they started posting '14/88' on their social media pages? People would protest, people will stop buying Coca-Cola, people will move on to other soda brands, and their stock price will go down. Cola-Cola's image would be tarnished that they may never recover from a stunt like that.
My hypothetical example with Coca-Cola applies to every private entity operating in the Western world.
The blame doesn't lie in the corporations though, the blame lies in Western society itself because most people living in the Western world object to neo-Nazis, white supramacists, doxers, stalkers, and other extremist groups which is why most businesses won't do business with the groups I just listed.
Why do banks don't want to do buisness with Kiwi Farms (or with any other similar community for that matter)? Because the banks don't want to be seen as entities that support stalking, doxing, and harassment. Why would Bank of America, Chase, Citi Bank, Stripe, PayPal, Mastercard, Visa, and so on support a doxing and stalking forum? Because the rest of Western society would give the banks a lot of shit for supporting such a business, that's why.
Jersh isn't just at war with the banks but he's also at war with Western society itself because Western society in general opposes Kiwi Farms and what they do.
Tl;dr it's horrible optics for any private entity to support an extremist group or an extremist movement of any kind.