[TorKlingberg]

They are still earning interchange, and profit from foreign currency conversion rates unless you watch out for it.

They can also earn money from retailers to promote them to you. Point hunters often end up spending extra to reach a rewards threshold, which can be very profitable for the retailer. It's also common to sign up for a card with great introductory rewards and then keep using it for years, because you get busy and forget to switch card every few months.