It's not a scam unless they are blatantly lying about something. Kiva itself doesn't charge interest rate instead their "field partners" do, which seem to be independent organizations that Kiva is partnering with. Here's information directly from their website:
The Field Partner collects repayments from Kiva entrepreneurs as well as any interest due and lets Kiva know if a repayment was not made as scheduled. Interest rates are set by the Field Partner, and that interest is used to cover the Field Partner's operating costs. Kiva doesn't charge interest to its Field Partners and does not provide interest to lenders. Kiva also gives Field Partners the option to cover currency losses.
They promise not to have field partners that charge an absurdly high interest rate:
Our Field Partners are free to charge interest, but Kiva will not partner with an organization that charges exorbitant interest rates. We also require Field Partners to fully disclose their interest rates.
I don't know what would constitute as an exorbitant interest rate in this case, but I'm going to assume they are keeping that promise as they also have open books by providing a list of all their partners with the interest rates charged.
If you click the various partners you will get to see statistics for how much has been invested through that partner, how good they are at collecting money and so on. Kiva also outlines exactly how high the interest rate is with the partner you are viewing, how it compares to other partners in the same country and how it compares to the average across all Kiva partners. Here's one example:
This field partner Median for MFI All
Peers in Country
Average Interest 12.50% 16.17% 37.03%
Rate and Fees
If anything (although I'm not exactly familiar with aid organizations or micro-finance) Kiva seem to be doing a really good job at being transparent by providing all of this information so readily accessible on their website. It is as far as I'm concerned the donors fault if they give money to an organization that uses it in ways they do not approve of, when the donor hasn't done at least basic research first.
Unless there's strong evidence to suggest Kiva is actively misleading people, and/or exploiting poor people for the purpose of economic gain, I don't see how they could be called a scam.