I think it's more like drinking milk past the expiration date ... or using a med past the expiration date. Or Installing a set of duracell batteries after the 'best if used by'.
"It will probably still work, but we, as a corporate entitity, are no longer responsible if it fails to act as originally promised."
So the US Military could only get it's money back if it was deployed and failed to kill anyone BEFORE the expiration date.
After that it's a matter of use at your own risk.
(although if the expiration date is related to long term storage, THAT could be a significant issue. Does the explosive filling of a bunker-buster become sweaty over time like other HEs?)