Of course they do. But more importantly they have to provide a product, at a price, that people will voluntarily come in and buy. To do that they can't charge $20 for a Big Mac, even though technically they are free to do so.
It's no different with any other business that has to compete for customers. If Comcast throttles Netflix too much then people won't use Comcast. Comcast may get Netflix to pay a little extra fee. And given how much of the bandwidth Netflix uses that's not really unreasonable. But even without it, Comcast knows they can't break Netflix for their customers or they won't have any. There doesn't need to be a government rule to enforce that. This is how markets work.
I've had VONAGE phone service for nearly TEN years.
I had it some two years before Comcast even offered it.
After Comcast began offering it and getting me to try to switch from Vonage to Comcast's phone service.. for nearly a year and a half I had a constant problem with being in the middle of a phone conversation and suddenly losing connectivity. This would happen 60% of the time I was on a call that lasted longer than 7 minutes.
I eventually found out through a Comcast technician that this was a Comcast tactic employed to get non-Comcast phone service users to switch to Comcast.
My point: Comcast had the upper hand of CONTROL because Vonage needed an Internet connection to pass through in order to work.
Comcast CONTROLLED that connectivity. My only alternative for connectivity at the time was AT&T's DSL, which IMO was inferior and cost nearly as much as Comcast.
By the way.. during this interruptive period I was deluged with emails, snail mail ads, and TV commercials trying to sell users, like myself, on Comcast's Internet phone service.
ALL of this happened during a period when there was NO NET NEUTRALITY. Between 2005 and 2012, five attempts to pass bills in Congress containing net neutrality provisions failed. Each sought to prohibit Internet service providers from using various variable pricing models based upon the user's quality of service level, described as tiered service in the industry and as price discrimination arising from abuse of "local monopolies enshrined in law" by some economists
Net Neutrality came into existence in 2015.