But people are paying these loans back so it has to help somebody. In real estate with hard money the juice may be very high (60% was usually the min.), and it can still be very beneficial to the borrower. APR in and of itself doesn't determine the value of a loan.
We are talking about very small loans. Anywhere from $50 to $1000 (as I recall the average being about $200). So say this same vendor (bringing in a TR of $.90/day) gets that $100 loan so she pays $150 in a year. Since she brings in $.90/day, and no longer pays equipment rent, she can bring in $280/year (assuming a 6-day work week). At the end of year she can pay off the $150 and still triple her take-home income. Even if the loan is for $150 and she has to pay off $225, she still comes out better off in that first year, and then the rest of her life is gravy.
$50 - 1000 is not a small loan for these individuals.
Since you appreciate Yunus's work, I would recommend looking up what he has said in the past two years re: micro-finance. It is not working anywhere near what you think.