Tanks are really expensive but are completely legal to own. Driving them around is another story i think.There are no federal laws governing the private ownership of flamethrowers or flamethrower-like devices. It's perfectly legal to buy them in 48 states and it's only restricted in Maryland and California, I believe. Using a flamethrower on someone is illegal, buying and owning one is not. It is also legal to purchase and own a tank, because this is America.
Also, didn't someone get arrested a few years ago because he used a flamethrower to clear snow from his front yard or driveway or something along those lines?