The desensitization of one ear by a single headphone played loud enough to cut through background noise changes the apparent location of sound sources. Open-air headphones do not cover the ears, impairing hearing, any more than goggles cover the eyes impairing sight or a scarf covers the nose, impairing the sense of smell. It is also fair to point out that headphones have practical advantages and legitimate uses for bicyclists, more so than for other vehicle operators. This is, after all, precisely why headphones are popular with bicyclists. Headphones are lightweight and require very little electrical power to operate, important advantages on a human-powered vehicle.
Headphones deliver sound to the bicyclist without disturbing other people.
In this context, headphones make it possible to teach safe riding, give route directions or relay vital safety messages over a far greater distance and more reliably than by mouth. It's because this eliminates the problem of feedback from loudspeaker to microphone. For the same reason, headphones make it possible for a bicyclist using a two-way radio to conduct a normal conversation, rather than having to shut off the microphone when receiving. Headphone laws are very rarely enforced. Many bicyclists ignore them.