They can always blame someone in their supply chain. And I'm pretty sure returns aren't permitted anyway. And they have guns and are on drugs and could even report you to the police.
The normal incentives for ensuring quality simply are not there. I don't think they know about Six Sigma.
Drug lords are much like the big telecom companies in the US: They do want customers, but their product has steady enough growth that they don't mind burning a few customers if it's convenient to their business model. (See, for example, all the hate AT&T has gotten over its handling of iPhone customers.)
