From the HP
website "If you don’t pay in full by that time, you can opt to pay a portion of the balance and interest charges will apply from the transaction date."
A credit card is not required but you will end up getting a bill just like any other you receive. If you fail to pay you will be charged with late fees and so on. I don't think HP
is counting on that as much as just being able to generate additional revenue from another one in a long line of innovations (food for sale on board, first to revise their whole pricing structure, no Saturday night requirement) which other airlines have eventually followed as a result.