Continuing @lulu's analogy: a town has low speed limits. They enforce the speed limits by putting in a bunch of speed bumps, signs, etc, but people keep speeding. So the local police decide to start using security cameras to catch speeding. They make a big announcement: "If we catch you speeding on video, we'll fine you," and then they start doing exactly that. They could go back through the archives and retroactively fine people, but the point is not to punish people, but to prevent future speeding. Hence the benefit of such retroactive action is minimal and punitive. Why bother? — Xander Henderson ♦ Mar 15 at 15:01