There may be a simple reason for this, as well as why no one has replied to you thus far: nobody "throws away" their water bottles. three points: 1) A good water bottle is something you hang onto; a bad water bottle should never be brought to or used in a race in the first place. 2) Just about every race I've been to has penalties for littering on the course, and they are enforced. This includes inadvertantly dropped water bottles. 3) My water bottles are insulated and cost $8-$10 apiece; not only do I stop, go back and pick one up if it drops during a race, I'm traumatized if I do lose one anywhere. I'm sure as **** not going to be "recycling" them at every race. Hope this answers your question.

I totally agree. When I take a plastic water bottle at a race, I take it home to recycle it, because I can't stand to see a trash can full of plastic bottles.