Why would visa/mc want to decline money? They love making money.

Because the money involved is peanuts, they don't want people running around trying to put them in the headlines as "enabling sexual exploitation", and a certain number of their internal people actually believe in the "cause".

Contrary to popular belief, corporations are composed of humans and do not reliably or mechanically follow only financial incentives. Nor are they always perfect in understanding their actual financial interests. Boycott threats are probably empty, but some people may not want to take the chance, or may have other motivations that cause them to overestimate that risk.