Almost every fish store I have been to sells non-aquatic plants as aquarium plants. In some cases the plants are non-aquatic, but can persist under water for months. If purchasers were warned, I could see a case being made for some plants (like Spathiphyllum, which is apparently good with very boisterous fish). It's irresponsible to see plants like that to unsuspecting aquarists. I was quite disheartened by my initial attempts at establishing plants (although salting the tank to deal with ich was probably the biggest plant killer).
It's one thing to sell your customers something that will never work. It's quite another to actually make them accomplices in lawbreaking. While I have not seen Hygrophila polysperma for sale in a fish store, it's available online. There are also reports that it is sold in pet stores. More disturbing is that fact that aquarium plant books published by reputable publishers say nothing of the fact that transporting the plant requires a permit, and possession in some states is a crime. Granted, there's no reason for a British publication to say anything about this unless they are producing an edition for the US market.