Charity and Races as Complements

[Robin Hanson]

Why do charity races make sense?  I can understand why people give to 
charity, and can understand why they participate in races, like running 
or biking.  But why are these activities combined so often?   Why limit 
who can give to a charity to who can run in a race, and why limit who 
can run in a race to who is willing to give to a charity?