Well, how about "standard practice" in the real world?
Read the rules for any contest run by corporations, etc. There's always a reminder that "entries must be postmarked no later than...."
Because in the real world, everyone knows enough not to rely on the speed and efficiency of the bloated bureaucracy know as the "U. S. Postal Service."
This is apparently too impenetrable a concept for politicians to grasp.
Still, delivery of mail cannot take place until the individual sending that mail enters it. If they wait until election day, it will not arrive until after election day.
But all of that means nothing. What we have here is the Democratic Party and the State legislature in the State of Maine violating a federal voting rights law and apparantly no one cares to complain, nor do they know where to go to complain. I would suggest that they go to a court of original jurisdiction at the federal level and file suit against Maine for violating "Motor Voter". Cite the George Bush case. Ask for $100 million!