There are a lot of political reasons behind the law outlawing growing hemp in the USA. It is said that it was made illegal because new nylon or man made ropes could not compete against the old fashioned hemp ropes that had been used for centuries, so hemp was made illegal to allow the man-made rope industries to flourish. There is a new kind of hemp being trialled that has very low levels of (THC?). It could be argued that this is not the traditional hemp plant and maybe that could be grown. Certainly "hempcrete" sounds like a very useful insulating material, that also removes carbon dioxide from the atmosphere and locks it away for the life of the building. I believe, however, that it is not as strong as traditional concrete and cannot take as heavy a load, so needs some kind of frame to allow it to be used for building.