I also wonder how often they do it, and why. You don't win any money, there is no incentive I can make out beyond bragging rights.
I mean, they aren't exactly footraces, right? It's an endurance test, IMO. Usually for charity, people usually participate for the exercise and show support for a good cause, or so I always assumed.
Winning seems totally arbitrary on one hand, but on the other hand I have to assume people do compete on some level, which makes the cheating more sensible, if you think about it in that sense.
I cheated once when I was a teenager at an online competitive game, so I'm not completely ignorant about the temptation of cheating. I'm used to people cheating at competitive online games, they're usually underskilled and looking to compensate, underage, just looking to spoil the fun, or some combination of those.
Maybe that's the intention here, to spoil the fun for those looking to compete. I suppose I'll never really know, but I can't help but ponder it whenever it comes up.