Alcohol is the common counter-argument but I think there is a significant distinction. Most people can use alcohol in a safe, controlled way, maybe get a little tipsy in the bar, but then sleep it off and turn up to work fresh as a daisy. In those cases where people overdo it and turn up to work p1ssed or incapable due to a hangover (or don't turn up at all) then they should rightly be disciplined - and I know of cases of that happening.
Most illegal drugs on the other hand, leave the users' abilities impaired for some significant time after use. I recall from RAFCAM briefings that cannabis use causes impaired coordination, reactions and decision making for about a week. LSD can cause all sorts of mental problems for years after use. Many drugs cannot be used in any sort of controlled way due to their addictive nature and will inevitably destroy the user and spiral out of control. Apart from these issues is the fact that a drug user is voluntarily a law breaker: is that compatible with the ethos and standards of military life?
I really do not think any sort of tolerance or leniency is the answer to the drugs issue. Such action seems to me much like the police and govt's policy of improving crime figures by decriminalizing everything and fixing the stats. It is ironic that these days booze and fags seem to be more of a crime than e's and 'erb.