They were found to not be machine guns per the statutory definition. So they can't arrest people for having one (unless a new law is passed). Prior to the BATFE's policy change they didn't 'approve the sales' more so they didn't bar the sales.

It would be elementary to make bump stocks illegal, because bump stocks are not firearms. Making bump stocks illegal wouldn’t cross the Second Amendment.

Correct. The issue was that the ATF tried to do an end-run around the legal process. Somebody in there did not watch that Schoolhouse Rock song about how bills become law... All that has to happen (federally, anyway) is that Congress must pass a law prohibiting them and the president has to sign it. But that's not what happened. The ATF -- under Trump's direction, mind you -- tried unilaterally to redefine an item that is not a firearm as a regulated firearm. What is and is not a firearm (and what is and is not a "machine gun" also) is already codified into law.

You can argue for or against unelected agencies having the ability to create new regulations with the force of law behind them without involving the usual system of checks-and-balances, but specifically in the case of the ATF they have repeatedly demonstrated that they are not able to use such a privilege in good faith. They would be (and are) exceedingly likely to use it as a cudgel to play these "legal yesterday, felony today" types of games with people so give themselves excuses to kick in doors and shoot people's dogs.

Various state laws already prohibit bump stocks. My state is one of them.

I've followed it and the first paragraph is spot on. But I'd argue something doesn't have to be a firearm to be protected, see Caetano v Massachusetts. It probably would also protect other items adjacent or necessary for the intended purposes of the amendment; like some types of ammo, parts, or accessories. If bumpstocks fall into that is another question though. Reading the tea leaves it doesn't seem like it.

But, it instead enacted a statute that turns on whether a weapon can fire more than one shot “automatically . . . by a single function of the trigger.” §5845(b). And, “it is never our job to rewrite . . . statutory text under the banner of speculation about what Congress might have done.” -pg 19

because it doesn’t make the weapon fire more than one shot with a single pull of the trigger.

So if I make a device that spins and presses the trigger for me, then it's not illegal, got it.

What’s to follow though? ATF or DoJ are not going to challenge any of the ruling, especially after this case. It’ll remain on the federal register, but stayed from enforcement indefinitely

See, you kid... buuuut...

https://youtu.be/Wa6OrbwXFWI

https://youtu.be/BOvoiXGUuYg

https://youtu.be/9WudGlIlqEk

Yup, all in how loose a grip you're comfortable with.

They've been doing that a lot, and for good reason. The whole gist of originalism, and the legitimate gripe conservative scholars have with substantive due process, is that during the latter half of the 20th century the judiciary usurped Congress' power in actualizing new legal concepts and theories by creating rights out of thin air. Even RBG admitted that Roe was on shaky ground, because in a representative democracy the only entity capable of making laws is Congress. If the law says "the sky is red", and that law creates obvious societal problems, it's not the job of the judiciary to say, "no, the sky is blue". The job of the judiciary is to say, "the law says 'the sky is red'" and that's the law that shall be enforced, and then to interpret the words if there's any disagreement about what "red" means. A functional Congress elected by a sensible electorate in a healthy democracy free of corruption (HA!) would see the obvious error and pass an amendment that changed the law on the books into "the sky is blue."

In striking down Roe, they clearly signaled that the only remedy would be Congressional action. From Kavanaugh's concurrence in Dobbs:

The nine unelected Members of this Court do not possess the constitutional authority to override the democratic process and to decree either a pro-life or a pro-choice abortion policy for all 330 million people in the United States.

RBG herself on Roe:

The ball, one might say, was tossed by the justices back into the legislators’ court, where the political forces of the day could operate. The Supreme Court wrote modestly, it put forward no grand philosophy; but by requiring legislative reexamination of once customary sex-based classifications, the Court helped to ensure that laws and regulations would “catch up with a changed world.”

Roe v. Wade, in contrast, invited no dialogue with legislators. Instead, it seemed entirely to remove the ball from the legislators’ court.

And Roberts in Shelby v. Holder (a decision which I loathe):

That is why, in 2009, we took care to avoid ruling on the constitutionality of the Voting Rights Act when asked to do so, and instead resolved the case then before us on statutory grounds. But in issuing that decision, we expressed our broader concerns about the constitutionality of the Act. Congress could have updated the coverage formula at that time, but did not do so. Its failure to act leaves us today with no choice but to declare §4(b) unconstitutional.

In short, the only suitable remedy to legal conundrums is a new and updated law. The fact that our democracy has deteriorated to the point where Congressional representatives are really just mouthpieces for interest groups and have insufficient dignity to even attempt overt moral judgments about how to form a more perfect union, explains why the judiciary keeps very clearly reminding them what their job is. Political compromise is hard and takes immense amounts of work and sacrifice, and SCOTUS is reiterating that they're not going to bail a dysfunctional Congress out by legislating from the bench.

The problem now is that in the modern global age we have plenty of enemies who have massive standing armies. China and Russia leap to mind. That sort of thing may have worked when America was physically isolated from outside forces by a several months long boat ride. Not the case anymore.

Abolishing everybody's massive standing armies would be a pretty good idea, but I don't foresee that happening any time soon.