Seems like you are missing the point a bit.
Because of the open registration policy of those instances and the many users they do not have the necessary tools and manpower to moderate harassing and offensive posts which go against their policy (which they can absolutely set as they want, since it’s their instance) so they block the users of those instances from posting and commenting for the time being.
So it’s not about isolation themselves just for the sake of being and exclusive club of people but because the moderators can’t handle the amount of traffic