I thought that it was illegal to import or distribute H. polysperma in all of the U.S. It's been banned from California for years. However, I think this is the only Hygrophila species (along with all of its variegated forms) that is banned. Most others should be fine. Bacopa monnieri, Cabomba caroliniana, Ceratopteris thalictroides, and Ceratophyllum demersum are also illegal, I believe.
I don't know about other states, but California can really only enforce this on big shipments. Inspectors do check LFS's, too. The law in Texas is harsh - you get fined PER STEM or PER PLANT. So if the fine is, say, $100 for H. polysperma, and you have 10 separate stems in your tank, that's $1000. At least this is what I've heard.
The easiest way to get a hold of plants that are no longer permitted in the state is to join your local aquatic plants society and something as common as H. polysperma is easily acquired at any meeting. Someday we'll have "plant-easies." Plant police will be doing under-water operations to prevent the distribution of moneywort. Hybrids will be developed to create "counterfeit moneywort" and the value of the true species will suddenly skyrocket. I fear the day is coming soon...