If we played their game...
We'd take all those envelopes with military ballots, or "find" them, and put a black marker over the postmark. Oops, no way to tell if they weren't mailed before the deadline. Every vote must count, right, and how dare you disenfranchise those who are willing to die for our country?
Don't be silly. If it isn't postmarked before Election Day, it's not a valid ballot.