It incentivizes Comcast to provide faster service for those who pay. It incentivizes them to provide
slower service for those who don't.
We can comfortably say that 99.9% of the services on the internet are not going to pay Comcast. The big players might, but there's an extremely long tail.
Thus, for most of the internet, Comcast encounters a conflict of interest. Its actual paying customers want higher speeds. But it has every incentive to reduce speeds for services that aren't paying for more.
I pay my ISP for access to the entire internet. If they screw with my access to a service simply because they didn't pay, that makes no sense.
If I'm paying $75/month for high-speed internet access, that means the entire internet, not just the parts they get kickbacks from.
I pay my ISP for my access, Netflix pays their ISP for their access, and we get to talk to each other. That's how this stuff works. Comcast charging Netflix is double-dipping, and is directly opposed to their supposed purpose of serving me, their customer.
(Hypothetically. I'm no longer a Comcast customer, thank goodness.)
Edit: I usually hate real-world analogies, but I think it could be instructive here. Let's take the USPS as an example of something in the real world that looks a lot like an ISP.
I can pay USPS $5.60 for a flat rate Priority Mail envelope, and they will deliver the contents anywhere in the country within 2-3 days. Doesn't matter who's getting it, and doesn't matter what's in the envelope as long as it fits.
Now, let's imagine that some company ends up receiving a lot of mail. Providing mail service is expensive, right? It takes a lot of trucks and postal workers to deliver all this mail to this one company. Surely they should be charged for what they're using, right?
Of course, this company says no. These people who are sending us mail already paid USPS. It's their job to deliver mail to its destination once they get paid to do so. Why should they get paid twice to deliver the same piece of mail?
In retaliation, to "convince" this company to pay, USPS starts slowing their mail. Those trucks and mail carriers are expensive, we need some of them elsewhere. You'll just have to wait.
And as a result, because USPS is demanding that this company pay to receive mail that I already paid to send them, my "Priority Mail" starts taking 4-5 days to arrive instead of 2-3.
I hope it's obvious that USPS is completely unreasonable in this hypothetical scenario. I see no difference between this and Comcast charging Netflix for service I'm already paying Comcast for.