Essentially it stems around your safety and the safety of other runners. If your music is quite loud, you cannot be aware of your surroundings and if someone is trying to scream your name - you won't hear. This also goes for when you are running in general - if your music is loud, you will not hear sirens, cars, or even people trying to warn you of possible eminent danger. At Somersault Events, they do allow music devices, but you are only allowed to have one ear piece in, allowing you to still be able to listen to fellow runners or event coordinators. I remember at the last event, an attendant was trying to get the attention of a runner, but they had their music blaring with both ear buds in use.
How do you maintain your safety? Be smart. Do not go out at night, wearing all black and have Lady Gaga blasting into your ears. Save that for the middle of the day, and if possible, try to wear only one ear bud because then you can also be aware of your surroundings. A company, One Good Earbud, offers 3 safe choices for athletes which fit securely and can withstand vigorous exercise!
Check out One Good Earbud for more information!