> So if you connect a device(i.e. a TV) to WiFi and there is a Netflix app that's downloading movies in background then Netflix has gained unauthorized access to the network

There are two main differences. Netflix is working on your behalf. Connections initiated from netflix are no different from connections initiated from your web browser or your email client. Your use of the internet, for you, is authorized. With a proxy you have no idea who is connected or what they are doing with that connection. That network activity uses you, but isn't by you, or for you.

Finally, netflix isn't likely to be hacking, attacking, spamming, or attempting to bypass bans or IP blacklists over that connection, but those are exactly the kinds of activities that shady residential proxies are used for. By using those kinds of apps you basically backdoor/bot your own computer, and now access to the internet through it is being sold to people who want to use it for things that they couldn't get away with when using a VPN or any other legitimate provider.

Depending on the sophistication of the wifi network you may be held liable and accountable for everything that takes place over that connection, including whatever nasty things the people who bought access to your proxy get up to.

If the wifi system isn't set up to keep careful track of who is using it and when (as is common in places like coffee shops), the liability will fall to the entity offering the wifi access, and if that connection is repeatedly used for activity in violation of the law, or the AUP of the ISP being used, that same entity will face the repercussions. Their internet connection may be temporarily suspended. They may have to start blocking certain ports and services, or deny access to specific websites. They may even be forced to stop providing wifi access to guests entirely unless they are willing/able to spend the money needed to log and monitor who is using their connection and respond to notifications of abuse over their network as well as legal requests.

Places like coffee shops that just want to offer free wifi to their customers without worrying about that kind of thing are only able to do that because a small number of local individuals are using their connection. Locals who presumably enjoy being able to the internet at that location and won't take advantage by abusing it. Individuals who are also typically on camera, in full view of employees and other customers.

That kind of very common set up is insecure by nature and those companies are putting themselves at risk by offering the service, but it mostly works because the access is time/location-gated and most people in the neighborhood are generally not criminals. Allowing criminals from all over the globe to access those kind of connections through your device is allowing unauthorized access, and is also being a dick, since you're basically shitting in the pool everybody wants to be able to swim in. It greatly increases the risk that the wifi gets shut down, gets more locked down, or the IP address gets blacklisted making it less useful for everybody who uses it, including you.