In the US, without net neutrality I believe it’s completely legal. I remember seeing a report on The Steven Colbert show about a year or so after we lost net neutrality about how Comcast deemed Netflix wasn’t paying them enough money so they throttled Netflix into the ground. This gave the appearance that Netflix services were crap in comparison to their own services like Hulu. About a month later they came to an agreement and Netflix paid up then magically speeds were restored to about the same as Hulu services.
I totally get what you are saying but think of the average user, they are not like most people on this site who know and are willing to fiddle with Tech. Plex is a media server, porn comes in all sorts of images, gifs, and videos which are all media files. If you already have a server set up, it’s as easy as adding a library and pointing it to the folder you want instead of signing up and configuring a whole new service that most users are not going to have the time or want to set up. Especially because before this push to share what everyone is watching on plex, it was all private anyways.