Coff, I can only imagine it's to let the camera see the pilot's face. Or so that the adoring crowds can see the pilot's steely smile during taxy.
Obviously no need for O2 at low level, but stupid not to have the O2 mask in the event of fire, fumes, ejection (part of helmet stability). Presumably they would have to put the masks on for transits - who knows what altitude you'd need for various reasons. Also the mask helps cut out cockpit noise when using RT and offers some (not much) protection against injury from birdstrike.
Seems pretty silly to me.