I'm not really sure, but since ma and pa were attached to him while casting the jutsu,technically he was also involved the process. They would probably have immunity from their own genjutsu. Being attached to the genjutsu user(s) is probably the answer, since later during the Pain arc, ma and pa said that Naruto would be caught in the sound genjutsu with Nagato since he was the distraction and was not near them or attached to them.
Maybe because the frogs were connected to Jiraia .