A) to make sure it came to a dead end.
B) the heavy metal concert across the street needed emergency special effects so he was lending his fists of thunder and lightning.

Less joking, the Marut doesn't care. They are servants created to enforce ONE specific cosmic law, not every law. They will only focus on their appointed law and accompanying task.

In general, lawful outsiders are going to support orderly existance, tempered at times by their own bents. A Modron is quite a substantially different creature from an archon or devil. Law is more useful to understand in this context as a style rather than a robotic malady. Some are completely logical, some are emotional (iirc even Primus felt fear in Modron March before he was offed), and above all they are generally capable of changing, fallen archon, rogue modron.