The internet is basically three components: Consumers, ISPs, and Data. The purpose of the ISP is to connect the other two, indiscriminately.
Bit of a simplification, don't you think? Sure, the ISP exists to connect the consumer to the data, but the data is not local to the ISP. The ISP has to have outbound connections to the data providers, and different data providers will need different connections, which may have differing costs.
The ISP has to determine which of those connections are necessary, and what speed to run it at. Your ISP may prefer to spend a lot of money on a fat pipe to the BBC if that is where the demand is, that does not mean they have to provide an equivalently fast connection to BobsBargainAmmo.com - they have to discriminate based upon destination.