Presumably, or allegedly, for legal reasons.
In the service of Her Majesty, we availed ourselves of the pure oxygen available in the safety equipment section for testing oxy masks to clear any lingering cobwebs from the night before.
This tradition applies just as nicely and successfully on the other side of the Channel, if I am to believe a good cockpiteering friend.
Perhaps oxgyen supplies should be left hanging around before crew go through security.