What decreasing drug laws? At the beginning of the 20th century, you could legally buy cocaine, in your Coke-a-Cola no less, smoke whatever you wished, and you could also own whatever firearm you chose without any federal permission. It's not just a coincidence that the first federal firearm law was patterned after the Harrison Narcotics Act. Very explicitly so in fact according to the testimony of the Attorney General before Congress when the NFA was being debated and passed. Both laws used, illegitimately IMHO, the delegated power to tax to regulate what was otherwise not within the delegated powers of the federal government to regulate and in the case of the National Firearms Act, to infringe upon that which the Constitution states "shall not be infringed".
It's also probably no coincidence that these laws were passed shortly after the repeal of alcohol prohibition.
Decriminalization and "medical" marijuana laws coincide with the recent rise in overall marijuana use.