It's immoral to tempt someone into harming themselves, especially when it also benefits you.

Payday loans tempt the weak with options that help in the (very) short run, but harm in the long run.

The very existence of payday loans encourages self-destructive habits among the poor. Why budget carefully when you can just get a payday loan and kick the can down the road to next month?

And since many payday loan customers get these loans every month, the loans are really just an additional monthly bill for poor people.