1. By enhancing the price, monopolists sold something for more than it was worth, which was against the idea of equality underlying commutative justice.
2. Exploitation in whatever form was against the precept of charity and brotherly love.
3. Monopolies were injurious to the commonweal, because monopolists not only increased prices, but also withheld supplies from the market and thus created artificial scarcity.
Adapted from Julius Kirshner, ed., Business, Banking and Economic Thought in Late Medieval and Early Modern Europe, Selected Studies of Raymond de Roover, University of Chicago Press, Chicago, 1974, p. 278-279.