If you're the kind of person that needs to go to a payday loan place, you're not going to be eligible for a 0% APR card - if you're lucky, you'd be offered a fully-secured credit card (meaning you have to give them money first before you can use it, and you can only use up to the amount you've given them) with a maximum balance of five hundred bucks, which you wouldn't have lying around anyways.

You get those card offers now because you're (presumably) employed to some degree and have a decent credit rating, so the card company thinks you're a pretty good bet to not default. If you need a payday loan, it's unlikely you have anything near a good credit rating, if you have one at all.

As for optimizing things, it depends on who you want to optimize for. If you want to optimize it for the credit card and payday loan companies, that's pretty much been done already - they have a very permissive set of regulations they operate under, and aren't short on profit margin. If you want to do it for the customers, that means you need to set up a financing service aimed at high-risk individuals, but you're going to have to mark-up some to make up for the default rate.

A good deal of people who frequent payday loan companies can't even get it together (for whatever reasons, both in and out of their immediate control) to get a normal checking account - you think that they'll be offered no-interest unsecured credit cards?