Plastic cups are not good protection for animals that use shells. These divers dont forcefully take the cup shell either. They’ll happily give up their plastic cups for the new shells being offered as long as they like it. Then the plastic cup is thrown away.

There’s a difference from scientists trying to make artificial reefs (which even then like the tire reef fiasco turned out to be a terrible idea) and random trash being picked up and used by the local wildlife.