Most important is not that it's robots, but what material is it that attracts only plastics, not myriads of other materials in soil and water. That is like saying, we have this magic filter, that only lets fast molecules through, so we use this Maxwells daemon in our robots, did you hear, ROBOTS! The main essence of the story is just glanced over, pff some plastic attractor, whatevs! But robots. And also, that is not even robots by definition.