Businesses have to pay for bandwidth to get data to the customer's ISP, but they generally don't pay the customer's ISP for the same bandwidth the customer has already paid for.
Netflix did not have to pay Comcast either. One of their ISPs Level 3 already had peering arrangements with Comcast to deliver the content. Instead of paying their own ISP, Netflix wanted to get free bandwidth from Comcast. There's a difference.
Comcast threatened to throttle customers' bandwidth, refusing to deliver the speeds they had promised. The data was available to Comcast, customers had paid for the service of delivering that data, but Comcast wouldn't provide the full service they had sold unless Netflix paid them more.
The deeper issue is that Comcast is lying to their customers, promising them more bandwidth than they are able to deliver, so when Comcast's customers wanted to use all the bandwidth they bought to watch Netflix, Comcast couldn't afford to honor their promises.
But Comcast has a monopoly in the markets they serve, while Netflix exists in a competitive market, so Comcast got away with it.
No ISP can guarantee speeds outside of their network. Once it leaves their network it's considered best effort. Comcast has about 30 million subscribers. If they were to guarantee bandwidth out of their network, and if every subscriber had a laughably slow 10Mbps connection, Comcast would need 300Tbps of connectivity to every single company and network. For this reason, every ISP in the world "throttles" in one way or another.
