Imagine you go to the grocery store, choose one apple (which is priced at 50 cents) and go to the checkout with this one item. The clerk then charges you 75 cents. "Why are you charging me 75 cents?" you ask. "50 cents for the apple, plus 25 cents to eat it" responds the clerk.
Customers paid Comcast for Internet service. When those same customers tried to access NetFlix Comcast throttled the service, rendering the service quality unacceptable. Comcast didn't block NetFlix: Comcast blocked their own customers, failing to provide the very service they had already been paid for.
Later Comcast blackmailed Netflix into paying for the same bandwidth, the bandwidth that Comcast's customers had already paid for.
It is complete bullshit and should be illegal. At a minimum Comcast should be sued in a class action lawsuit.