I don't think it's that great for them to eat, but during the past 5 years I've progressively given up on trying to prevent my rabbits to chew things that aren't good for them (except if it's poisonous or really dangerous, of course): paper, cardboard, wallpaper, electrical cord, the floor, clothes... they tried everything and everytime I take something away from them, they find something else. The litter tray is something they never tried. Was it new? It's hard plastic, I would have thought it was pretty complicated for a rabbit to take a bit off of it. If the littertray is damaged or maybe there was a problem with it in the first place, making it not as sturdy as it should have been, you might want to try replacing it...