I agree completely, there have been many articles written on how Americans break laws all the time without knowing it (No, I'm not saying that buying/selling/doing drugs falls into this umbrella) and the government cannot possibly prosecute everyone from every crime. This allows the government to pick and choose who they prosecute. Lets imagine the following scenario which is very similar to the one above.

1. Person P smokes marijuana by themselves, doesn't sell, doesn't buy large amounts, "Personal Use" 2. 3-letter-agency ABC is pretty sure person P smokes through some text/call/email/web browsing/etc but has bigger fish to fry 3. Person P is an outspoken critic of government G (or just a supporter of something government G is not a fan of) 4. Government G runs a search on person P through their database and find that ABC is 85% sure this person smokes marijuana 5. Government G tips off local police that they might just want to stop person P's car on X date 6. Person P is now either imprisoned which will greatly hamper their efforts or scared into shutting up and keeping their head down

Now let smoking marijuana be swapped out for some other crime, something that "everybody does", like downloading a song/movie/app, or maybe it's even more minor but they decide to "Make an example of you". The example zeteo gave is very good but most people can write it off with "Well he WAS committing a crime" but if the end goal of the first stop (the "Foot in the door") was actually to censor the person then I think people would see more of an issue.