Monetary boundaries are quite common in law. The "grand" in the video game titled Grand Theft Auto refers to stealing of a car worth more than $5,000. Stealing of a car worth less than that is simply theft (of an) auto. There are a lot of factors that go into a decision not to prosecute, but there are many petty crimes that go unpunished and unprosecuted. (There are also some that are prosecuted, but that doesn't disprove that some aren't.)