It sounds like the lady in the story collects shells mainly on St. Thomas - which is not within the National Park so it is legal to do so. On St. John, you are not supposed to take shells from national park areas.
Tons of shells and other "sea souvenirs" are collected at the STT airport as guests leave. You might not have been checked or even asked but I've seen folks have to give 'em up. And they don't bother to ask where you got 'em. (But the best shelling is at Brewers and after a storm).