It is a great question, and one I usually address with a simple and quick answer. Marijuana is generally harvested and distributed from and/or to the U.S. While cocaine is generally harvested and distributed from countries outside the U.S. (mainly South America), basically all of which use the metric system. [As a side bar, as of 2006 only three nations in the world have not switched to the metric system; United States, Liberia & Burma]
So it would be easy to say that possibly marijuana being weighed in pounds is only because the United States uses the customary standard. In fact, in Amsterdam, where marijuana is legal, it is weighed in grams and not fractions of ounces like in the U.S.
And for anyone who has purchased marijuana in their life could probably tell you that there are 28 grams in an ounce. A simple conversion between the metric and customary systems.
But it could be argued that cocaine is a more worldly drug that is shipped to more locations than just the U.S. so it would be inefficient to weigh it in a format that only fits one country.
In fact, there is a great argument that drug dealers and distributors dictate the weight format purposely for where it will be distributed.
And when law enforcement stumbles upon some of this contraband, they follow suit; 99% of the time listing how many pounds of marijuana and how many kilograms of cocaine were apprehended. There is no conversion given to say "25.3 kilograms of marijuana" or "30.5 pounds of cocaine". They keep it in the traditional format that drug dealers and users would be accustomed too. That is an interesting concept as well. One might think that it would be in law enforcements interest to keep everything in the same weight format. But then again with different penalties for different drugs, the weight format may not be a huge issue. But it would be very interesting if the laws for the different drugs used different weights. Such as a felony offense is for 10 pounds possession of marijuana but 5 kilograms of cocaine. Under the same legal system, why the different weight formats?
It would be my strong estimation, that the U.S. is the only country in the world where marijuana is weighed in the customary system, and it warrants it, because nearly a third of the country admits to using marijuana at least once in their lives. That is over 100 million people using the measurement system they are used to. And it would be my estimation that the U.S. has one of the largest audiences for marijuana further suggesting it would be worthy of a separate measurement system.
Drug lords in South America seem to me to care less about easy distribution in the U.S. due to weight format. They know they have the best of their product and basically the only mass production of it in the world, so they can format their weight any way they wish without worry of losing business.
Hope that quick sum up helped answer the question. It has a similar tri-answer to that of what is most important in real estate; location, location, location....
