What if I charge you $6 because you order so many hot dogs that I have to find another distributor of raw hot dogs and hot dog buns? And charging $3 would mean that I'm taking a loss?
That doesn’t really apply to this debate. Netflix isn’t sending data back and forth through the ISP’s network just for their own fun and their own benefit. They send the data because the ISP’s client specifically asked for it and is paying for that bandwidth. They are also likely paying for far less bandwidth than the low bitrate Netflix is using. TV streaming is also arguably one of the main reason why the end user bought a high speed internet connection in the place.