The version I learned is that it's a historical artifact from dial-up Internet.
You had the phone lines from the phone company (natural monopoly, common carrier), and completely separately you had an ISP with a modem bank hooked up to an Internet uplink (competitive, information service).
Then you had the Telecommunications Act of 1996 (I turned twelve that year!), where the phone company had to allow the ISPs to use the phone lines at wholesale rates. So you get ISDN and later DSL with that same split between who controls the wires and who controls the Internet uplink.
Then you get cable Internet. Which does not use common-carrier wires (I assume this is related to not-so-modern TV being entirely one-way). And which end up classified the same way as potentially-unbundled Internet that runs over the phone company's wires, despite being controlled by the same people as the wires it runs on.
Then for whatever reason all the independent DSL providers disappeared (or at least shrunk to where they can't afford noticeable advertising campaigns). I don't know if the phone companies found a way around having to share their lines, or put less effort into maintaining lines used by independent ISPs to discourage people using them (I know I heard this suggested back when it was new, no idea if it was reasonable or just a conspiracy theory), or just out-competed them based on there being a single bill and a single point-of-contact (and no finger-pointing) for problems. I guess it can't be the first option, since I've heard that a few parts of the country actually have more than 2 choices (cable company & phone company) for Internet service.
You had the phone lines from the phone company (natural monopoly, common carrier), and completely separately you had an ISP with a modem bank hooked up to an Internet uplink (competitive, information service).
Then you had the Telecommunications Act of 1996 (I turned twelve that year!), where the phone company had to allow the ISPs to use the phone lines at wholesale rates. So you get ISDN and later DSL with that same split between who controls the wires and who controls the Internet uplink.
Then you get cable Internet. Which does not use common-carrier wires (I assume this is related to not-so-modern TV being entirely one-way). And which end up classified the same way as potentially-unbundled Internet that runs over the phone company's wires, despite being controlled by the same people as the wires it runs on.
Then for whatever reason all the independent DSL providers disappeared (or at least shrunk to where they can't afford noticeable advertising campaigns). I don't know if the phone companies found a way around having to share their lines, or put less effort into maintaining lines used by independent ISPs to discourage people using them (I know I heard this suggested back when it was new, no idea if it was reasonable or just a conspiracy theory), or just out-competed them based on there being a single bill and a single point-of-contact (and no finger-pointing) for problems. I guess it can't be the first option, since I've heard that a few parts of the country actually have more than 2 choices (cable company & phone company) for Internet service.