I know the urban legend you speak of. I personally believe its kept alive for few reasons, and that it's origins are from Mexican bricks.
My idiotic imaginative guess:
You got to imagine what happens when you compress that much weed into a brick while it's still wet. It's really amazing that more of it doesn't start fermenting than actually does. Cannabis seems much better at holding off fermentation than regular alfalfa hay, and I have never understood that because of the structure of both plants. Anyone who is going to do something like that to herb is going to buy into something like curing it in the ground too, as this also offers a great deal of stealth. When the stuff is down here, crossing the border, or slightly before it is going to, it is fucking hot! 104 the other day, and I'm not even near the really hot parts where even plants don't live. You probably get an automatic 10-20 degree difference just by burying the shit underground and a natural seepage of moisture because of the dry ground. Those both seems like good reasons to bury it instead of hiding it in a tree or something, for grand smuggling purposes.
P.S. When we were kiddos, the rumor was that it was buried underground, and that's what gave it the red hair. Hence, people still say Mexican red hair, when it's a good bricked up sativa down here. I know I'm not the only one who heard that shit as a stupid kid.
I'm just guessing and hangover rambling though.