Yep, it is a "law thing". However - can you honestly say that you, yourself, have not broken any law within the last 24hrs? I bet most of us have somewhere along the line, consciously or unconsciously. There's so many areas of law, not just criminal, that it's hard not to.
The world we live in is pretty full of morons, which I believe laws set out to protect us from. Unfortunately, laws also make criminals of decent people who're doing nothing wrong but trying to live their lives as best they can. Obviously, Inspector Plod won't usually take that as much of an excuse if you're caught doing something naughty!
Cannabis can be picked up in your blood for 6 weeks, sometimes more, after you've last had it.. The number of people banned because of blood tests showing positive for it are tiny though, mainly because the effects on someone who's used the drug before driving are so slight and the Police have got much better things to do than random stops on motorists. If the effects were anything like what booze does to you, then I'm sure the number of bans would be far higher.
I think the main issue here is that the drug laws in this country, and most others, are completely outdated, along with many peoples attitudes.