"How do you explain that it's wrong to support these terrorists by buying their product? "
The same can be said of the labor practices in production of chocolate, but you aren't going to get any moral outrage there either.
We would have cocaine in the US with or without FARC. Before farc and before the US pushed the colombian government into going after the cartels, it was being produced and will continue to be produced no matter who in running colombia or the traffic.
You aren't going to get drug users to alter habits based on a long-running civil war in colombia's spillover effects.
Explain to me how eating chocolate is the equivalent of smoking crack.