These are legal somehow in USA? Can you please trace how and prove it with links?
In 1986, they stopped accepting new registrations for automatic guns. So there's a fixed supply of legal automatic firearms that normal people can own.
Read more here: https://en.m.wikipedia.org/wiki/National_Firearms_Act#:~:tex....
Why do people want them? Is it just a hobby, or is there some utility I'm unware of?
Outside of the military they have no utility whatsoever.
Another nit pick: the approval is not some sort of interview or questionnaire as of why you need it. There is so-called ATF stamp (i.e. $200 for silencer) than you need to submit altogether with FBI fingerprints. These days mojority of ATF applications is registered thru "trust", instead of an individual. This creates all sorts of issues for LE in case they want to run extra background on you etc. So 90-something % of people chose $50 ATF "trust" that Sheriff cannot technically decline over the other option which is petition as individual and getting close to 100% chance of denial. The biggest hurdle is not some LE not liking your tattoo, its that ATF has a year long backlog. So you pay for your FA $25,000 today but its only delivered to your local licensed gun store, and they won't let you pick it up until ATF sends them document with green light. That take sup to 1.5 year these days. You can come and touch it and take few photos but they will not allow you to load it of course, or to take it out (including shooting range).
Finally, as OT mentions fixed supply, a so-called lower receiver of a fully automatic gun, a piece of metal not much bigger than a door handle (which does not even have any fancy engineering) is literally the most expensive piece of metal that you can buy on American soil. Some examples in extremely good shape go for $35,000 these days (check gunbroker dot com)
~~The Assault Weapons Ban only applied to new guns.~~ ATF policy change, not AWB. Existing guns were grandfathered in.
Guns have many interchangeable parts. The part of the gun that the government considers to be the "legal definition" is a part called the lower receiver. You can change almost anything about a gun as long as you keep the lower receiver and comply with any other laws.
So if you buy an old MAC-10 lower receiver that was manufactured before the new ATF policy, and add an adapter [1] that allows it to be combined with the non-lower parts from an AR-15, you can create a completely legal Frankenrifle with mostly new parts!
This isn't even the craziest thing you can do with gun loopholes in the USA. "Pistol braces" [2], originally made for war veterans with amputations to shoot small rifles, have pretty much obsoleted restrictions on short-barreled rifles. "Ghost guns" [3], i.e. lower receivers that have no serial number and do not appear in any database, are easily made by anyone with access to CNC tools (including a few hours rental for a few hundred bucks). 3D printed pistols are becoming a thing [4], although they are not very durable.
1: https://aandsconversions.com/2018/11/25/the-next-big-thing-f...
2: https://www.sb-tactical.com/product-category/brace/
3: https://en.wikipedia.org/wiki/Ghost_gun
4: https://www.reddit.com/r/guns/comments/jzik44/magdump_monday...
You are right, it was mostly a ban based on superficial features like the much feared bayonet mount. But there were two bans based on magazine capacity.
It did ban magazines with more than 10 round capacity. It also banned semi-automatic shotguns with more than 5 round fixed magazine or a detachable magazine when in combination with folding stock or pistol grip.
