Some non negligible percentage of people might choose the one that only works with most websites, due to some other reason, such as they have reduced prices.
That non-negligible amount of people on this other service, could still be leveraged by that ISP to strongarm websites.
An ISP grows a large user base by doing aggressive price competition, then starts shaking down services, rate limiting their traffic if they don't pay up. The services are over a barrel and pay, after which the ISP's users don't see slowdowns and have no reason to switch. Meanwhile they use some of the shakedown money to lower prices and get more users. Smaller ISPs don't have as much leverage to extract the danegeld so they can't compete.
It makes perfect sense to prohibit that business model as an antitrust measure.
Yeah, we’ve already seen this in the Comcast/Netflix debacle and Deutche Telekom vs the world. In fact a lot of large incumbents do this.
> Meanwhile they use some of the shakedown money to lower prices and get more users.
If things were even so rosy. The shakedown money is used for larger profits and monopoly positions to get more users.