"Hauling home 90 lbs. is pretty brazen, but if there’s a really by-the-book security person screening you, make sure there’s no trace of sand stuck in your sandals, swimsuit, or hair!"
I think you were just exaggerating for the sake of humour, at least I hope so. Why shouldn't security be by the book, and no, they aren't talking about a bit of sand inadvertently clinging to your clothing. But this isn't at all limited to Sardinia, nor with the large amount taken. It's against the law to remove anything from beaches in many parts of the world. But too many people think the rules don't apply to them and " what harm can a few shells and/ or a small bag of sand do?" But multiply that by thousands and thousands per year and the damage is obvious.