Why does Jacob even suggest that the men might believe that God justified them in their actions? The answer lies in the subtle difference between the acceptability of finding the precious ores and the condemnation of the costly apparel. Jacob notes that they are to be blessed in the land, and the men (as well as has Jacob) would assume that finding the ore would be a sign of God's favor. Combine that with the serendipity of Nephi's ability to teach them to do something with it, and you have a God-ordained predisposition to a favorable trade balance.
The problem is, of course, that the men are justifying their actions that have led to the wealth, and Jacob is condemning the effect of that wealth on their attitudes to their fellowman. The wealth is not unjustified, but the assumption of superiority, and the establishment of classes within Nephites society is damnable.