maybe this is urban legend... but i think the weed thing goes back to the days when people were deciding what to print newspapers on. Hemp paper was real popular and was going to be used instead of tree paper - and that's when the traditional paper-makers started to spread rumors about "reefer" causing people to go on axe murderer sprees and what not. Hemp was actually lauded as a high quality crop by many of the founding fathers, including but not limited to George Washington and Ben Franklin. You can do a hell of a lot with it, besides paper and smoking the buds you can make clothes, rope, and about a million other things.
Its just some cultural bull**** remaining from the 1820's that weed is illegal. These days the status quo is perpetuated by alcohol lobby, if weed was legalized their business would go in the crapper. Cotton, Paper industries also are threatened for reasons discussed above.