Slavery abounds in The House of the Scorpion. The brainwashed and trapped Eejits are the most obvious example of enslavement, but pretty much everyone in El Patrón's household is a slave in some form. No one is free to leave or live their lives without El Patrón's say-so. No one can question him openly or go against his word. Plus, if you think about it, Matt's entire body (namely, his young, healthy organs) belongs to El Patrón. It would be easy to think that these forms of enslavement are a result of El Patrón's dictatorial rule, but the scary truth is that slavery is widespread in the world outside of Opium, too. Even in Aztlán, which doesn't appear to have a dictator like Opium, the Lost Boys are slaves to the will of the Keepers, and must do forced labor in order to be clothed and sheltered. No one in this world is safe.
The fact that eejits are people captured trying to cross the Mexican-American border relates to the current immigration issues on the border between the United States and Mexico.
Farmer uses the character of El Patrón to show us that drug lords are essentially slave owners, who use various forms of slavery to keep their drug empires running.