Once sufficient bandwidth is in place, it costs an ISP nothing
Bzzz! Hold it right there! What is "sufficient bandwidth" and is it ever in place? What if twice more of your subscribers have signed-up with Netflix — the company is enjoying amazing growth of subscriber base? What was "sufficient" two months ago no longer is and you have to spend real money again. In this regard bandwidth really is like tangible goods.
Charging the streaming customers for downloading much more than others finances the further increases in spending.
More generally, however a private company wishes to charge its customers should not even be a matter of public debate — much less actual regulation. The only legitimate role of government here is to encourage other private companies to compete — competition being the best judge of both the possible and the affordable.