Flamethrowers are legal in the USA???
All Musk was selling in a propane torch. Not that good a one at that.
Propane torches are used about every where in the world. Usually, I admit, with much smaller heads and ports.
But you can get larger heads or buy torches such as weed burners quite easily.
When you go to longer range flame throwers you are generally talking about some sort of liquid or semi liquid fuel being used. Impractical devices with a number of safety issues so that they are generally not legal for sale simply on safety issues.
Outlawing them is a bit silly really. They are too big a pain to maintain and fuel to appeal to any sort of "market" of any size. Designing and building one that is not primarily a threat to the person using it is also pretty tough and requires some real skill at building even if you have the knowledge to do it.
And then what can you actually use it for?
A legal definition of a "flame thrower" for the purpose of making them illegal would be pretty tough. A lot of hot rod show cars throw a pretty good flame. Rocketry as a hobby is legal and they use a number of different fuels. Any burner for glass blowing works by throwing a pretty good flame and are pretty much identical to the burners for propane/natural gas forges. Every really good farrier usually will have a propane forge on his truck. Large forges using propane or natural gas will throw an extremely hot flame several feet if you took them out of the forge "oven" containment area.
Most grain drying "bins" are heated with LP because it is more dense than propane and they have huge diameter flames that will reach out several yards if ignited outside the heat exchanger chambers.
The nozzles for fuel oil furnaces do the same thing. They are pretty much the same thing as the burners for coal fired electrical generators as they both have an orifice for the fuel to be injected into a larger head where the fuel oil or coal dust is mixed with air for burning.
All of those types of burners are available. They are also not that hard to produce yourself. The larger they are the easier they are to build.
Making them so that you have a portable unit that a person can carry and not be a bomb is the tough part. Building one and mounting it on a vehicle would actually be easier in a lot of ways and would not take a whole lot of research to do.
Expense and a reason for building such a contraption is also a bit tough. What would you do with it? Light your barbeque pit? Better be a damned big pit and be used very often to make that worth your while. And even that could be lit easier with other methods.
If, on the other hand, you wanted such a device for an offensive weapon then having it outlawed would not be a real problem to you at all. But you are going to have an extremely impractical weapon. If you run into "opposition" all they need to do is spring a leak in your system and you will be the center of a fireball. This was the fate of many troops using flame throwers in warfare.
Note the lack of such things being used currently in the many "conflicts" going on.
Basically there just isn't any real reason to out law them.
Things like the one Musk was selling are a problem. Really aren't any good for use at parties. They are really identical to the thing Nubie is using for a weed burner. Note that he isn't using it to remove weeds that pop up in planters in his house. Or using it to get rid of "dust bunnies" under his bed. There is a reason he is not doing that.
Some people use them for melting ice on walkways. This is pretty silly if you stop for about 5 seconds and think about it but they do that sometimes. Generally makes for a much smoother coating of ice unless you heat the concrete enough to dry it out. This is going to use a lot of propane and will make the concrete prone to cracking when the warmed part expands but the lower, buried part is still at 25F.
Pretty much a limited use tool that is inexpensive for the purpose intended.
The ones Musk was selling at, I think, $500 a pop were silly. Not sure what the fuel container was on them at all but couldn't be very large. And it didn't look like it was nearly as well made as far as flame quality goes as one of the normal weed burners. But it would definitely burn your house down or at least set some of your furniture on fire. That would make for an exciting party.
We actually have a weed burner and use it sometimes for just that. We hook it to a 30lb propane tank which is not something you really want to lug around very far. But you could hook it to a small propane cylinder that could be in a sling from your belt or shoulder.
Is also handy, assuming you are confident you have no fuel or lubricant leaks, for heating up vehicle engine compartments in sub zero (F) weather. I have used the branding oven (for heating branding irons) with a 100Lb propane tank and a couple of sheets of scrapped metal roofing to heat tractors that some idiot (me) forgot to plug in the night before so I could go feed in the morning. Doesn't take long to heat things up but you have to make sure that you don't burn the equipment shed down.
That burner is about the biggest portable burner I have ever been around. And it is practical for what it does. But you can go through about 100LB of propane per 200 to 300 calves branded depending on how well made the burner is and how well designed the "oven" is. Say about 4 hours of burn time. Not something you fire up for entertainment purposes unless you are made of money or someone else is paying for it. Or you are an idiot.