It dates back to the constitutional ban on "ex post facto" laws. Meaning, the government can't retroactively make something illegal. Which is a good thing, IMO.

So, for example, it's illegal at the federal level to manufacture machine guns (and I'm not going to get into a gun debate or nuances as to what defines a machine gun--it's just an example). But a machine gun is legal as long as it was manufactured before the ban went into place. Because the government can't say "hey, destroy that thing that was legal to manufacture, purchase, and own when it was manufactured."

This concept is extrapolated here to say "The cops didn't do anything illegal at the time. We have determined this is illegal behavior now, but we can't use that to overturn police decisions that were made when the behavior wasn't illegal. In the future, cops won't be able to do this."