The remaining monsters were still watching the situation and not helping their comrades fighting. Because of the murderous intent they remembered oozing out of Jon, they stood still and besides he was watching them.
So, they didn't move, but if they were provoked like the ones fighting were, they would also attack.
And as for what Jon thought of as he watched them, he didn't think too much on the fact that the remaining monsters didn't gang up on them. They would die if they tried it; it was that simple.
Seeing the girls fight made him start noticing something odd. He was at level 35 when he arrived and fought with the monsters.
And even with that, he was still able to dent the body of a beetle, but here the girls were struggling.
Especially Cynthia and Jenny; they were at levels 34 and 35 too, but they couldn't do the same thing he could.