I haven't got an opinion really on whether I believe them as not much evidence from either side has come to light yet - the newspapers seem to have already made their mind up on the matter though (their party drug history has nothing to do with this). I am relieved that Peru doesn't hold a death sentence for drug trafficking.
However, I am worried about the way that these countries handle mules. IMO, the wisest thing to do would be to follow them on towards wherever they were planning to travel to, and arrest whomever was on the other side. A highly publicized arrest with a harsh sentence hardly does anything for the war on drugs. Cartels and Drug rings are far more powerful than people seem to want to give credit to, and there could be the fear over whether they could have infiltrated the police. IMO they are going for the wrong people - these are not the masterminds.
These rings could have your ID, phone, address, photos of your family - would you really trust the police(especially in a place you don't know very well) if you've been viciously blackmailed? Some people might, but I think the fear and adrenaline pumping through your veins could lead you to make some irrational decisions.
Not to mention that these girls are pretty young, and could have been groomed for this simultaneously - being told that 'no-one actually cares about drug trafficking in Peru - the Government gets a cut' or 'the Police are in on it too' - whatever, something that could have made them think it was safer to attempt it.
Or maybe they just did it for money