Indeed, water lettuce is illegal to sell in Alabama along with water hyacinth. They are an invasive species in our state. I've seen water hyacinth clog up water ways, boat ramps, ect. on the warrior river.
To answer the question, it is illegal to sell, but I don't believe its illegal to posses. I have seen places still sell it in state, but it's just a matter time before they get caught and a heavy fine imposed on them. I did have some water lettuce in my outdoor pond, but the cold weather wiped it out.