This rule is all about lawyers and insurance companies. How many
accidents have you seen occur from a person running a 8 or 9 minute
mile pace in a marathon occur? I can understand if the runner is a
top finisher that headphones wouldn't be the best thing because of the
speed they are running, but for runner wearing headphones that finish
in the range of 4 hours...please!!!!!!! I am registered for the
Cleveland marathon in 2008 coming up here and I see they have banned
headphones from the race. I'm still wearing mine. This country is
supposed to be free. If this continues to happen more I will stop
registering for these races and just run as a bandit runner.

http://www.iht.com/articles/2007/11/01/sports/run.php