[whoknew1122]

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."

[chipsa]

The government has totally said “destroy the thing that we said was legal to manufacture, purchase, and own when it was manufactured.” That was the entire point of the bump stock ban, which attempted to reclassify an item that they had previously said was not a machine gun into a machinegun, and therefore illegal to own (and was always illegal to own, so they weren’t going to compensate people for them either).

More strictly, machine guns aren’t banned by the federal government, but rather you have to have paid a tax to own it, and they’ve banned paying the tax for gun made after X date. If they decide to ban the ownership, grandfathering is not guaranteed.

[michaelt]

> Because the government can't say "hey, destroy that thing that was legal to manufacture, purchase, and own when it was manufactured."

Actually that's a totally normal way for bans to work.

If a state decides to ban a book from school libraries, the libraries don't get to keep the books on the shelves because they already had it.

The ban on ex post facto laws merely means that, if a ban on a given book is passed today a librarian can't be punished for having it on the shelves yesterday.

Grandfathering in exceptions is just politics - make a bitter pill easier to swallow for the people most impacted; delay the costs of any remediation; deal with historical/museum pieces; and simplify enforcement.

[Nevermark]

> If a state decides to ban a book from school libraries, the libraries don't get to keep the books on the shelves because they already had it.

That isn’t comparable.

Comparable is a ban on printing that book. Which would not be a ban on existing already printed copies. It would only be a ban on new copies.

[Suppafly]

>It dates back to the constitutional ban on "ex post facto" laws.

Not really, that's not now constitutionality works with respect to the government. Ex post facto is when the government wants to act against you, not when you want the government to behave. They use new decisions regarding constitutionality to undo previous decisions all the time, they just don't want to in this specific case and are using the "well they would have been able to get a warrant anyway if they had known they'd needed one" to justify it.