Take for example, if say Verizon decided to charge more for bandwidth to Netflix... if Netflix said "no" and went with another provider, then Verizon's customer's would suffer from worse access times to Netflix.
Verizon has the advantage in that they have a huge customer base that no one wants to piss off Verizon. So it cuts both ways. Bandwidth becomes not a cost at this scale, but instead a moat.