Kilos & Pounds

I was reading a Prensa article the other week about a police seizure of over 100 kilos of cocaine in two separate operatives. It made me curious. Why do the US and Guatemala use the metric system when referring to quantities of drugs while they use the English system for most everything else? (Well, Guatemala is actually English/metric mixed depending on if its volume, weight, or distance, but trying to explain that is for another post.)

But thinking more on this kilo/drug connection, I realized it isn’t for all drugs. You never hear about the police catching someone with a kilo or grams of marijuana. It’s pounds and ounces for weed.

Is there actually some good reason for this, or is it just custom? I suppose I could ask one of the local traffickers for his insight, but really I’m not that curious.