Earlier quoted context omitted.
Dont you think users with malintent abuse the platform irrespective of whether a card is on file or not?
Credit cards act as a "cost" of sorts. Credit cards are a limited resource, it is not free to acquire more credit cards. By requiring a valid credit card you are basically relying upon the verification that credit card issuers do to prevent unlimited abuse.
I constantly get free offers on the mails, plus there are services to generate temporary card numbers for throwaway