It's to do with the amount of electromagnetic radiation emitted. EM radiation interferes with aircraft systems, most notably radar. CD players were banned by international law a few years ago, but have now been accepted as safe for use on board aircraft. Some airlines, however, may not be taking the risk. Rick Webber, London, UK My CD player interferes with both my radio and portable telephone.
Most electrical equipment emits some kind of radio signal.
This may have some effect on an aeroplane's navigation or communication systems or whatever.