Courts apply the laws. For that, they need to read them, to understand them, and see how they match with the facts being judged.
Sometimes the laws and the facts are clear and there is no place for doubt. If a law says that I must not cross a line and I was found strolling ten meters beyond the line, there is no doubt that I broke the law.
But some times it is not so clear cut. What if my feet stayed in the correct side of the line but I flexed my body over it? What if I just passed my hand briefly over the line? What if I crossed the line to help someone who was having a heart attack?
Also, there are questions that arise from the conflict of different laws. If a law says that I must comply with police requests, what happens when a police officer tells me to cross the line?
To sort out those cases that were not clearly defined by the laws, the courts interpret the laws, often taking into account not only the letter but also the intent of the law... why did the legislative forbid that I cross the line?
And of course, when multiple persons (even in the case of trained professionals like judges) make their own interpretations, their conclussions might differ. In these cases the judiciary can make its own common interpretations (jurisprudence *1) of the laws, that are to be followed by all the courts.
When the issues are related to things not clearly defined in the law those interpretations will not contradict what is written, but if the issue is that a law is conflict with other laws then the courts may decide that parts of the law are invalid.
In this particular case, if there is no minimum then you probably are in trouble: "92% of the bills were positive for cocaine with a mean amount of 28.75 ± 139.07 micrograms per bill". Drug detection tests can detect incredibly small amounts, so a minimum is needed so that the law cannot be used to jail anybody; if the law does not set one it is not unusual for the courts to try to define one on its own.
As an aside, for the sporadic user case, most drugs leave traces of its use that can be used to detect past use (you can detect THC metabolites in the blood up to six months after using it). Nails and hair are particularly useful, as they store chemicals extracted from the body chronologically (the further away from the body the compound is found, the more time it has happened since it was present in the body). It would be rather strange for a regular user not to be able to prove it as part of its defense(*2).
*1 The way of establishing jurisprudence may vary from jurisdiction to jurisdiction.
*2 And of course, breaking the law always carries some risks...