Sheila Terry (Fine Gael)

I want to raise a brief technical point of information. Bills should be expressed in plain English, where possible. While I am aware that two negatives make a positive, why do we say, "it shall not be unlawful"? Why not say, "it shall be lawful", or is there some legal term about which I do not know? It is a simple matter, why not put it in the positive form?