Having spent quite a bit of time in the back of Horizon airplanes, I can assure it was the flight attendants selling credit cards, and not anyone on the flight deck.

As I understand it, Visa pays the flight attendants a certain amount for every application that gets processed (along with bonuses for "top performers"), so there's a financial incentive for them to hawk the cards, despite the fact that most of the passengers just ignore the pitch or find it annoying.