>The stuff stays in your system too long

The evidence of the stuff stays in your system much longer than the impairment. There is also the issue of "tolerance" and or how the amount of substance present in urine or blood relates to the supposed impairment.

>So now you effectively have no way to prove whether or not someone was breaking that law.

But you can still perform an objective test to determine if someone is currently impaired. They need to design and validate a motor skills and reaction time test. I'm not sure why anyone cares more about why someone is impaired than they do about whether someone is impaired.