Kiva, the organisation itself, does not make any money off of the interest charged. When you make a loan, you have the option of tacking on a donation to Kiva onto your payment (I never do.) The interest you see on each loan is charged by a field partner for operating costs, and Kiva has strict regulations on disclosing that information to lenders and borrowers. If you don't like their rates/whatever - then choose a different field partner to work with.
I have loved Kiva since it's inception. Every time we have one loan paid back, we make another. Annabel and I read through several profiles before selecting the woman we support. We choose women specifically to donate to because in many parts of the world, we are still second class citizens. It is more difficult for a woman to obtain a loan & operate a business, among other things.
For those taking out the loans - having a loan with interest keeps people independent, and gives them a sense of accomplishment - fills them with Pride. It isn't a charity handout, kwim? :)