I can configure whatever credit card I want to use under Apple Pay. Apple Pay itself is free. So what is really the problem?
Nope... I tried this when Apple Pay was introduced with a random card issued by my bank and it failed. Only until Apple Pay was introduced in my country, plus another few months, a couple of local banks had a contract with Apple Pay and then it was possible to use Apple Pay as a debit card. No credit card options.
And while Apple Pay seems to be free for us users, it is a paid service for banks (thus we do pay for this in the end)