The Dark Lord had many different methods to punish his followers when he felt they deserved it. One of his favorites, as a method to control both parties, was to insist that spouses conduct the harshest punishments against each other. Also a twisted test of loyalty, it showed who was more important: their spouse or their master. If it was even suggested that a husband or wife was going easy on their spouse in their punishments, they would be expected to receive double. Hermione could understand his reasoning. Even though they were required to propagate their magical bloodlines, Lord Voldemort wanted their ultimate loyalty to be to him, not to each other. Much easier to crush potential rebellions in that instance.