slightly modified:
The comparable scenario would be that your electricity company not only provides electricity, but also owns an appliance manufacturing chain. The electricity company charges you a different rate depending on which manufacturer's appliances you buy. Additionally, if you buy their appliances, then you get free electricity for those appliances.
The above scenario doesn't wrap in that it's not just a lower payment rate but, due to artificial caps on monthly usage, can lead to a situation where the only option becomes "use the ISPs preferred music/video/etc provider, or don't use that type of service at all".
edit:
P = (A * R) + (B * R') + C
Where:
P is the final bill a user pays (or data charged towards a cap)
A is the amount of services that are provided by the ISP/Provider/Pipe
R is the rate that is charged for the provider's own services
B is the amount of services that are provided by a third party
R' is the rate that is charged for the those third party services
C is a base payment amount (or zero in the case of considering P a data cap)
Assuming that equation, then the lower R is, the higher R' is, and lower P is, the more likely a consumer is going to choose ISP services.
So, if R is 0, then the ISP can slowly lower P (when considering it as a data cap) to be basically zero. Thus pushing out an ISP competitor services (or making new services not even viable), as the user is highly incentivized to use ISP services. The user is also incentivized to use ISP services, even if they are worse, otherwise be penalized with data cap usage or higher rates.
When R == R', then the ISP is a "dumb pipe", and competes to provides those services, and has no other incentive to do anything other than provide transit.