Rightly so. I mean maybe it’s too strict (i.e. you’re totally sober but still will be penalized for having anything show up in your blood) but in general driving under THC influence is as bad as driving on alcohol, you could get yourself and others killed.
It's really not. There have been tests done by a government transport regulatory agency (in an EU country), comparing THC, alcohol and sober driving.
While alcohol promotes reckless driving (i know what i'm doing, i've got it, even if i don't), driving under THC is different, people are much more careful, even compared with the sober state. Slower driving, less incidents.
Alcohol makes you kill yourself and others, takes only few hours to get out of your system. THC makes you drive slower and more careful, can be found in blood days or weeks later (with no effect on cognition or faculties).
You're right that driving under any influence is (probably) not safe. But this seems like the same kind of stupid like testing your hair for drinking alcohol few months back.
Whether magistrates would look at the levels of THC in your blood and be sympathetic to an argument that you had consumed a long time ago (and thus were no longer under the influence) is another question. I tend to think magistrates would side with the law as written in most cases, if the law explicitly prohibits driving with any amount of THC detectable in blood tests.