I believe the cabin-crew have a freedom to decide if they charge for the headsets or not. I have been on a domestic United flight which departed 2 hours late, and they announced by PA that headsets were free today to compensate the trouble.
On the other hand, I was on a TWA-flight were the F.A. announced "If you don't buy a headset and then we catch you using your own headset, we will still charge you a $5 entertainment-fee."
Maybe the TWA-cabinstaff get half their salaries from the headsets-payments??
Of course you can escape off a plane with headsets (tell the collecting FA's you didn't get them, or you dropped them on the ground and can't find them), and bring that one on board another flight (esp. when their system is non-walkman-compatible, a double-plug thing).