R’ Dovid Feinstein writes that the Jews were expected to plunder the wealth of the gentiles because of mida kineged mida (“measure for measure”). After all, Haman’s decree (Esther 3:13) included gentiles plundering the valuable of their Jewish victims.
However, the Malbim points out that, as opposed to Haman’s letters, these letters did not imply that the plundering was to take place after the enemy was killed out. Rather, they only had one day! This is because Haman gave plenty of time to plunder in order to help motivate the hordes. Mordechai, on the other hand, did not need to do this since survival is the greatest motivator.
Class Participant YML suggested that if this letter were written by Achashverosh, it is possible he only gave them one day out of his anti-Semitic desire to give the Jews less than what they were entitled to receive.