Why are online garden cataloges allowed to sell invasive species?

For example here in NH there are certain laws about the sale and purchase of invasive plants, shrubs and trees. It is illegal for me to buy or sell these certain ones with in my state. Does anyone know why I can just get online and purchase them for my yard if my state finds them so invasive they have to create laws to inhibit the spread of them? I do see that there are SOME restrictions as to the sale of certain plant life to certain states, but the restrictions do not go as far as the invasive species lists.