I guess that depends on your definition of a flamethrower. To me, a flamethrower is primarily a weapon. And what you are describing is a roofing torch. Googling it now, I can’t seem to find anyone calling them “roofing flamethrowers”. Flame gun — sometimes — but never actually a flamethrower.
So the legality is probably an issue with not being specific enough with the tool/weapon differentiation. I don’t think actual flamethrowers should be legal.