And that business model produces market concentration.

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.