Difference is that when it comes to driving on public roads, the police (the law) don't take the same view. If a roadside test turns out positive, you're done, end of story no matter how long ago you had done something like smoked a joint. There is a growing school of thought, which has allies in both the legal and medical professions, that the roadside tests are more about punishing people for actually having illegal drugs in the first place than they are about getting those who are too impeded to drive off the road.
The USA and UK refuse to have the road side tests that we have here because they have found them to be unreliable. Yet the Australian Government had no problem in putting them into use.
I mean, you can get caught with marijuana in your system up to a month after smoking a joint. Is someone in those circumstances more impeded to drive than someone who has a blood-alcohol level of 0.049? Yet the one just a poofteenth under the 0.05 limit is legally allowed to drive away while the other is immediately taken off the road and basically has the book thrown at them. When you look at it like that it becomes less about being too impaired to drive and more about punishing people for possession and/or use of illegal drugs. Alcohol is legal, marijuana and meth aren't.