If you're really concerned about proof, just go to a hifi store and turn on your mobile hotspot without a password - I bet you could get a TV somewhere to connect with minimal effort.
https://web.archive.org/web/20190521054343/https://www.reddi...
The reason there are likely so few reports is because a)open wifi networks are very rare b)you can't really tell that the TV has done it, until it shows its hand (as in the case above, notifying the user that it auto-updated itself.)
If it’s not showing you ads, it might not be connected. It could still be connected and performing all sorts of shenanigans without necessarily showing you any ads.
When we have proof of one manufacturer connecting to an open wi-fi network to send its snooping data, we’ll have a a reckoning on our hands. I guarantee it.
Wireless networking is something I usually avoid in a technical capacity, but this seems like something worth messing around with (and reporting back on).
I'm not saying they wouldn't do it but I'd prefer some proof before judging them (without having to shell out to be the guinea pig).