Can you even prove that one was driving under the influence? If you haven't smoked a lot and they do some tests, couldn't you easily argue that you were high the day before, since it takes time to get THC out of your system?
They really didn't think this legislation through. I could have come up with a more detailed law. They're leaving it up to the police officers to use probable cause to do a blood test. And like you said, weed doesn't play by alcohols rules. You can smoke a week before and still fail a drug test, yet be as sober as the officer testing you.