Who said anything about it having the game? There are tons of the helmets already made and ready for the game to go in, so it is pretty reasonable that a few would get leaked out and sold. Happens with everything. Hell, Halo 2 had the entire game leaked months before it came out.
I haven't seen the ones in stores yet, but I wouldn't be surprised if they used the real things for the displays.
Ya, my uncle works at Pixar and he get DVD's way earlier than when they come out. Maybe this guy works at bungie or something. I dunno, just taking a wild guess.