The Pharisees seem to be holding an unflinching line on purity laws (hence their comment about the disciples washing their hands), a clear connection to their strict adherence to the law. Jesus, however, points out that a law about honoring parents receives some flexibility to preserve the spirit instead of the strict letter of the law. Jesus himself came to BRING that spirit, so trying to subvert it, even its presence in the law, is to miss the life that Jesus came to bring.