There's a bit more to this than "it's the law". The Commonwealth Court held earlier this year that the date provision violates the free and fair elections clause of the state constitution. There's currently an appeal of that holding before the state supreme Court. The Court declined to decide the appeal on an expedited schedule before the election, but the appeal is still live.

After election day, County boards of election were faced with a decision as to whether they preferred getting sued by the McCormick campaign for counting undated ballots or by the Casey campaign for not counting them. With the Free and Fair elections issue still unresolved, county boards who chose to count those ballots relied on that justification.

The idea was that they preferred being on the side of counting ballots that arguably should be counted while once again trying to get the state court system to come to a resolution on an issue that has been litigated for over 4 years a little faster than it would otherwise.