the inherit magic of the moon

The Scarterran moon does more than look pretty and regulate the tides. Zarthus loves to reveal hidden evils and sometimes moonlight does this literally. Remember the first Pirates of the Caribbean movie where moonlight revealed the undead nature of the crew of the Black Pearl. Some monsters are exposed by any moonlight. Some monsters are only revealed by the light of the full. Werewolves (and were-other animals) can shape change at will, but the light of the full moon makes them look like hairy monsters regardless of what forms they are in. Most lycanthropes change into their war form anyway because they are going to attract hostile attention anyway.
Magical disguises are harder to maintain during the night of the full moon. This is not an insurmountable obstacle. Most Metamorphs only suffer headaches during the full moon. Even though King Drosst is not wearing is disguise of his own free will, he still gets a headache every night of the full moon. Is it any wonder why King Drosst hates Zarthus ’ followers so much.