This is one of a handful of police procedures that are ripe for abuse. Any officer wishing to justify a suspicion can claim that they “smell marijuana” and search a vehicle, home, etc. There is basically no way to contradict them. It’s not like we have smell recordings.
Another good example is field sobriety tests (walk on this line, count a number of steps, etc.), which have been shown to be highly subjective and inaccurate even when done correctly. Policing is maybe the last modern discipline that ignores evidence-based best practices.