Airline provide it 'free'?
I suppose just like the bottle makers gave it to the airline for free, and the O2 suppliers provided their bit for free, and the safety inspectors provided their time for free, and the equipment makers provided their certified equipment for stowing the O2 bottles securely, and the handlers loading and unloading (and securing any residue) all of the above for free.
Oxygen under pressure is a 'dangerous good' and cannot be boarded without significant (and costly) checks. Can you imagine what would happen if a bottle was mishandled inflight and turned itself into a torpedo? It could, or more likely would, endanger the aircraft and everyone on board.
No, I don't want airlines providing this for free. They might be tempted to cut corners.