Post hoc, ergo proctor hoc - It means "after this, therefore because of this". It's a classic logical fallacy.
Those are interesting statistics, but there are a lot of other possible causes for the decrease in violent crime including more/better trained police, increased incarceration times, a better economy, etc.
My guess is it's due to the large number of criminals who are incarcerated - an argument that has been out there for some time to explain the declining rates of violent crime in recent years. Which reminds me of the oft-cited statistic (from lefties) about the US having the highest rate of imprisonment (up there with Russia and China), and how this is supposedly a baaaaaaad thing, indicative of some horrible flaw in our society.
Everyone likes to manipulate the numbers for their agenda.
Lies, damn lies, and statistics indeed.