Why did William Lytle Schurz called the galleon trade the most oppressive phase of the Spanish domination in the Philippine islands?

It was during this period that the natives of the Philippine islands was mostly oppressed for the benefit of the Spaniards. Natives from islands as far as Cebu were brought to manila to serve their polo. these men, farmers who could not afford to pay the amount to be exempted from polo were forced to work in the mountains of Cavite and other provinces, cutting timber, and drag these logs to the harbor to build the ships. they did all this work with no pay and sometimes, they weren't sent back to their own provinces (Cebu, Mindoro, etc).

These timbers were used for the galleon trade which was controlled by the Spaniards. The galleon trade lasted for 250 years and during this time, the indiOS suffered much but gained little.