Most drug use starts out as
“recreational”, whereby initial
use is stymied by the risk of
getting caught, and being charged
with posession of an illegal
substance. Dependency is gradual.
One doesn’t become a heroin
addict overnight. Take away the
risk of getting caught, the more
that are going to use.
An example would be a married
guy with three kids, with a good
job, car, and house, likes to
party on the weekends. His buddies
coax him into trying a “hit”.
Where he would have reservations
while the “hit” was illegal,
and he might lose all he has
due to a possession charge. That
threat of charge is no longer
there, freeing up the temptation
to use.
Take away the charge of
shoplifting, the more who would
steal. Take away the charge of
poaching, the more who would
take game, at whatever time,
for whatever reason.
His buddies coax him into trying a “hit”. Where he would have reservations while the “hit” was illegal, and he might lose all he has due to a possession charge. That threat of charge is no longer there, freeing up the temptation to use.
The grave intrinsic harms remain. To repeat: Is the illegality of heroin or meth YOUR primary reason for not using them? It's way down on my list.