With a casino, this might be true because the players play against the house!
I think though that with a poker room, in order to make a good amount of money from a player, you need to have action going on there. And, the fact of the matter is, that there are only a few poker rooms that always have action going on. So, therefore, eventough you might not get as many people refered, you will make more of the people you did refer when sending them to a big poker room, where there are always games going on!
But, correct me, if I’m wrong!