Because the law itself enables mail in voting to begin with and has a provision that only the legislature can change the rules under which mail in ballots are valid. If it is changed by anyone other than the legislature such as the local county office, the law enabling mail in ballots becomes invalid. As for an example, provision A enables mail in ballots, and provision C nullifies provision A if certain things happen.

The law dictates all valid mail in ballots must be in the county office by 8PM on election night. This was summarily changed by the PA Supreme Court, violating the law.