What is the theme of the poem "The people of Tao-Chou" by Po-Chü?

This poem describes an annual presentation that used to be made to the Emperor of China by a number of dwarf slaves from Tao-Chou, where people only grew to be three feet tall. These offerings were made until Yang Chen became governor, at which point he declared to the Emperor, "among all the things that live in Tao Chou, I only find dwarfish people, not dwarfish slaves." The poet concurs with this assessment, saying that the dwarfs could not be a "natural product," as this is dehumanizing and does not take into account the fact that these were people being separated from their families and lives. The Emperor, moved by Yang's declaration, prohibited further shipments of dwarfish people as slaves, and the poet goes on to explain that children born in Tao-Chou were sometimes given the name Yang in memory of this event. 
The primary theme of this poem, then, is that of slavery and how it relates to personhood, but we also find elements of the importance of ancestry, and of memorializing history, possibly to guard against repeating it unnecessarily. The people of Tao-Chou "still enjoy this gift," and the poet seems to suggest that we should all appreciate the struggles of our ancestors in helping us achieve a position in life where our personhood is respected.