Why doesn't Antonio think that the duke will forgive the forfeiture?

Antonio says that the Duke cannot disturb the rule of law that the city's business relies on. Nobody will trust doing business in the city if the rule of law is broken for Antonio.

The duke cannot deny the course of law. For the commodity that strangers have With us in Venice, if it be denied, Will much impeach the justice of his state, 30Since that the trade and profit of the city Consisteth of all nations.