Using trade preferences to promote non-trade policies is controversial. It's not completely clear to what extent this is permitted under WTO rules. And regardless of the legality, it can be irritating to those countries on the receiving end of these demands.
In addition to those questions, there is the issue of whether it works. Does conditoning trade preferences on labor, environment, intellectual property, human rights or drug eradication policies actually promote changes in those policies? In the case of the last one, the answer may be "yes, but only a little bit." This is from a U.S. ITC report on the Andean Trade Preference Act:
In 2009, ATPA continued to have a small, indirect effect in support of illicit coca eradication and crop substitution efforts in the Andean region. According to U.S. government data, net land area under coca cultivation decreased substantially in Colombia in 2008, but it increased in Bolivia in 2009 and in Peru in 2008 (most recent data available). Alternative development programs in these countries continued to provide the infrastructure and job creation needed to generate export sales of new or improved legal crops—such as bananas, pineapples, hearts of palm, coffee, and cacao—which are eligible for duty-free treatment under ATPA or NTR provisions.