Money was established for exchange, but interest causes it to be reproduced by itself. Therefore this way of earning money is greatly in conflict with the natural law.
It's very dangerous to mix up the words natural and habitual. We have been trained to be quite habitual at communicating in ways that are quite unnatural.
If a person is stupid, we excuse him by saying that he cannot help it; but if we attempted to excuse in precisely the same way the person who is bad, we should be laughed at.