Rob, much like you, I've taken water from some pretty crazy places. That said, I doubt we're representative of the average runner. Also, in a race like Ed's, I'd hazard a guess that most of the water was taken at the finish line, not on the course where you just grab and drink without thinking about what you're taking.
Honestly, I'm not sure how runners would react to reusable cups. That's where I see RDs being concerned. A lot of being a good RD is conveying a positive image of the race. While reusable cups could convey a positive image in terms of lessening the race's impact and keeping entry fees low, there is a potential for a negative image to form. The RD would need to consider the cost/benefit analysis on this. All I'm trying to express is that there could be an understandable concern from the RD's perspective that could make a recyclable/compostable alternative seem to carry less risk.