They're not unheard of in cities. Hotels and shopping malls have them. Some residential ISPs will also broadcast a public access point using a slice of each customer's bandwidth allocation...
When I had Comcast internet, I could get on those, but it required a network profile to actually connect to the internet. That said, I wouldn’t put it past TV makers and Comcast to collude on using this to allow backdoor network exfiltration.
TV makers would have to collude with every ISP for that to be worth depending on.
Not to say it couldn't happen - but it wouldn't be a backdoor thing, it'd be very up front. All that's needed is for ISPs to create/join some SSO-like standard that lets you use ISP credentials as smart TV credentials (e.g. Your Xfinity account is automatically your Vizio account, sign on with your Xfinity account to get free streaming deals now).
I feel like if TVs start doing that, they'll display a network ID and signal bars at the top of the screen like cell phones do, so the TV maker doesn't get blamed for ISP problems.