Paid headphones on domestic flights

On coach class long domestic flights, why do they make you pay $4 for the headphone
to watch movie? Considering the cost of a ticket is going to be $400 or so, do you
think people will care if the ticket is $408 roundtrip? The logistic of having the
flight attendants walk up and down, handle money, etc, is not worth the work,
or is it? Or are there some strange rule that make this necessary?
Some royalty thing with movie studios?