It's not really a capacity issue at all, because as you say, NF's bandwidth isn't very high compared to the pipe people are being led to believe they're actually paying for. It's more an issue of A: ISPs overselling their endpoint connection capacity, but that's not really a problem because netflix bandwidth is scaleable (quality goes down if the connection is slow), and ISPs could upgrade their hardware, except they don't want to spend the money. They prefer to just make money instead and not invest as much as they should have to, considering peoples' reliance and expectation of the internet keeps increasing.
And B - and this is the big one: ISPs do this because they CAN. Like I mentioned, they like to make money more than they like to invest, so if they can make money both from their customers (who are actually paying for everything they download - bandwidth over the internet is fking cheap these days), AND by taxing netflix, then they will do it. And nobody's stopping them. The american FCC even legally endorsed protection racketeering over the internet not long ago. That's extraordinary, and utterly offensive to anyone with an ounce of standards and integrity.