St. Thomas comments upon usury and how it is a sin against nature thus:
To take usury for money lent is unjust in itself, because this is to sell what does not exist, and this evidently leads to inequality, which is contrary to justice.
In order to make this evident, we must observe that there are certain things the use of which consists in their consumption: thus we consume wine when we use it for drink, and we consume wheat when we use it for food. Wherefore in suchlike things the use of the thing must not be reckoned apart from the thing itself, and whoever is granted the use of the thing, is granted the thing itself; and for this reason, to lend things of this kind is to transfer the ownership. Accordingly if a man wanted to sell wine separately from the use of the wine, he would be selling the same thing twice, or he would be selling what does not exist, wherefore he would evidently commit a sin of injustice. In like manner he commits an injustice who lends wine or wheat, and asks for double payment, viz. one, the return of the thing in equal measure, the other, the price of the use, which is called usury. (STh., II-II q.78 a.1 resp.)
But for Dante, it is a sin against art, and this is something explained by Virgil to Dante in Canto XI. I, however, (and in researching it have noticed I am not alone in this) find the explanation difficult. At the risk of extending too far what Dante may have meant and, at worst, missing the point altogether, I offer here a contemporary consideration of usury as a sin against art.
To produce something is to use those things God has given us and apply our own reason and work to it, thus making something that is useful to us or to another. Most often our livelihoods are based off of this. We make something, sell it, and use the money to buy something else we need.
Now, there are a great many politicians who base their livelihood off of producing nothing, but off of taking and redistributing other people’s earnings, often by way of taxes (but in other ways as well) to give to another. They also, in their salaries, take some off the top. They therefore produce nothing; they add no art, no product. They do nothing but charge (this is their salary) to redistribute money. They thus charge for the use of money not their own, produce nothing, and yet claim to have given something and done a service.
In this way they sell something that does not exist. They sell a product to those who they deem needful, and in doing so reduce what would be helpful to those people. They tax a business to help these people, all while these people are laid off from that business since, due to being robbed (excessive taxes are hardly different than usury as a type of robbery), these businesses lay off the very employees who think they benefit from the “product” given them by the government.
St. Thomas calls something usury because it is to charge for something that does not exist separately. You cannot charge for the wine AND the “use of” the wine. It is acting as if you have two products to sell when you really only have one. It is deceit. Is this any different than to bring forth policies that pretend to create more goods but really do nothing of the kind (All the while billing the people for doing so by means of a salary and, very often, exempting oneself from the policies of practical usury you put in place)?