In a free country, you usually do it the other way around-- that is, conduct is presumed to be legal unless there is a statute proscribing such conduct. (It is hard, though, to make the argument that the same presumption would apply to conduct that is malum in se, but that is not the case here).

For example, there are plenty of places in I can park my car in the country where there is not a "Parking Permitted" sign-- the absence of a "No Parking" sign or any other regulation means parking is permitted there.