Let's say that Amazon lends money to a high volume seller. And we know that Amazon is going to take a cut of sales. But, let's say, hypothetically, that all of this is a wash, in the end it ends up netting Amazon zero direct profit. Has Amazon lost money on this effort then?
Consider the indirect benefits to Amazon. By increasing the amount of business flowing through the site they further cement their brand and they increase the network effect of using the site. They bring more customers to the table who then become repeat customers due to a good experience on the site. Or they bring additional business as people buy related or unrelated merchandise on the site, perhaps in the same order. And they increase the potential customer base for buying Amazon gift certs. All of these things tie together and help bring Amazon more business.
Now, in reality Amazon is going to make sure it makes money on these loans and on direct sales as well, but it doesn't have to make a very high margin on such things because of the indirect effects I mentioned which provide additional margin.
As an ex bonds guy, I can say for sure that the lending market isn't close to efficient. One anecdote - the Fed has extended a ton of credit to banks (large and small) in the last 4 years. They've also been concerned in the process because those banks have hoarded the capital to cover loan default losses.
Small and Medium sized businesses are often unable to get capital because a. there's far too much friction in the application process, and b. banks often use the same metrics to evaluate large and small businesses, but oftentimes the credit of a small business aligns better with the credit of it's proprietor, than it does with any other factors.
This one's easy: because the lending market isn't even close to efficient.