Markets work for you and me:

I just checked in for my flight, and was given the option of inputting my credit card in case I want to buy snacks on the plane.

This is brilliant.

I rarely buy snacks on a flight. They are expensive, and the flight is usually not that long. But I don't know right now if I'll want something tomorrow, and this allows me to decide on the spur of the moment tomorrow without having to bother with digging out cash or a credit card while scrunched in an airplane seat.

Is this a dirty trick by the airline to get passengers to buy more snacks. No, unless you call reducing the total cost (including inconvenience) of a product a dirty trick.

Probably the biggest gain for the airline is not more sales, but ease of operations for their flight attendants. They don't have to make change (and spread germs to other passengers from handling literally filthy lucre), or burn time running credit cards instead of seeing to other passengers' needs.