venting from the rear end
Not everyone who drinks alcohol or smokes tobbaco is dependent on them (although some are), they are priced to maximise return.
Not everyone on a day trip to Calais is a smuggler.
If illicit recreational substances were legitimised not every day tripper would become a smuggler.
Medicinal benefits of opiates are well proven, medicinal benefits of marijuana are difficult to prove because the authorities prevent research
As for the 'Slippery slope' argument, not every person who tries illicit substances becomes a hardened recidivist criminal however it is a fact that every junkie/criminal and murderer there has ever been has been born, perhaps we should stop all people from breeding, that'll cure it.