People like to usher them out of the water, either because they are trying to help other people not get stung, or because they just enjoy it, or want to seem brave, or something. It turns out that you can touch them on the top without getting stung. The only dangerous parts are the tentacles, but those are plenty dangerous, I hear. All of this results in some guys in their teens and twenties grabbing jellyfish (which I gather are quite heavy, based on their size and and waterlogged-ness, besides being quite slimy) and hauling them out of the water. Once they get them onto the beach, the kids take over, throwing sand and rocks at them. I'm not sure this last part helps.
Here are some jellyfish we saw at the beach last weekend in various stages of the process. There were a ton of them: