Hot rocks in plastic containers of water may not cause damage for the same reason you can set a paper cup or paper bag full of water on an open flame burner. I think the theory is that the water keeps the container's temperature below ignition...below melting in the case of the plastic mug.

FYI, I have tried both the paper cup and the paper bag on a burner on my gas stove. Both worked...kinda. The heat from the burners was sufficient to ignite the top of the cup (that part that was above the water line) and it burned down to the water line and spilling started to occur. The bag didn't burn but it did leak from the start. Things were getting kinda sloppy on the cooktop before I finally gave up and took the bag off the burner. At that point the water in bag had achieved face-washing temperature and the bag had no charing/burning.

FB