This is considered normal:
But this isn't:
Much like talking to yourself in public, singing when you wear headphones breaks social conventions and makes you look like a crazy person. It's okay to sing along when you catch a song playing on the radio, but the activity becomes verboten once you pop on the headphones or earbuds. Your horrible voice is expected to remain private.
But the reason why I don't sing out loud when I wear my headphones? I don't need to advertise my presence to others when I'm jamming to Rod Stewart on the cardio machines. (And this is coming from someone who likes to break into dance routines in the middle of stores.)
Barb the French Bean