I probably wouldn't care about them allowing the kids to listen to music. For some kids it helps them focus. For example, my son gets easily distracted by "innocent" noises made by people around him. He is overly aware of noises from another room when trying to do homework, or someone tapping a pencil 10 feet away, or someone rustling paper... whatever. So earbuds and music could be a huge benefit to a student who has trouble with such distractions. Many classes now have time for reading built into them (the last 15 minutes of language arts class, for example, might be used 3x a week for reading AR books).
BUT, ipods don't just carry music. They also have calculators, movies, games, etc.... lots of ways to distract students FROM their work. And ways to cheat, too.
So... depending upon the usage, I think it should be up to the discretion of the teacher to allow those who find it helpful to bring and use them.
ETA: Just wanted to make a note-- the note says it is optional. So opt not to, if it doesn't work for your kids. Or if you are concerned about loss (theft or otherwise).
My daughter is in 6th this year and she can work fine with or without noise/music. My son is in 9th and he does better with it. His Literature teacher actually plays music in the classroom. (And he isn't playing Bach). Could I complain that it is old 80's rock music? Yeah... but why? My son prefers stuff other than Muzak when he is working. It works for him.
The thing is, what you have posted about is OPTIONAL. Your kids don't have to participate and any other parent that thinks it won't help their kids don't have to allow them to participate either. And I don't see how an ipod being listened to by a neighboring student is more distracting than a bobble head or Kooky pen or whatever. It is just a thing. Maybe I live in an affluent area (not) but by middle school MOST kids are exposed to and have devices like this. There aren't many kids who will be entranced by someone having an ipod or MP3. It's old news for them at these ages.