It's not a failing of the drug testing regime, it's a failing of the person using the drugs to understand that they cannot be under the influence of any substance drug prescription medicine or alcohol while at work on a mine site.
I totally agree moon, I for one don't want to be heading toward a bloke who smoked 5 joints last night with his mates and this morning he is in roaring down the Pacific Hiway in a B Triple, or operating a 500 tonne crane or a 100 tonne Dump Truck on a mine site after sharing a joint for breakfast...because some fool legalised it!
If one law or rule is relaxed you can bet your bottom dollar another law will come under fire...the defense Lawyers will have a field day... Everybody's metabolism and mental capacity is different...laws have to made to meet the very lowest denominator.