I think there's a difference between charging 30-40% APR (double what a crappy bank charges for a credit card, approx) and charging 4000% APR like Wonga.
Also note we're talking about different but related businesses - ludicrously overcharging for payday loans is one thing, ludicrously overcharging for hire-purchase is another. The second is less risky, assuming people don't trash their own furniture/possessions.
Finally, no, I am not certain an ethical payday loans business can exist, but I am certain an ethical loans business can exist. It's all a matter of gradually finding a limit, where people stop ranting about how unethical it is.
(to clarify - I am not actually a populist ethicist, I was merely speaking in practical terms.)
Or is the cost of doing business where 9/10 people default (mostly because you charged them too much) just that high?