There's a thread on Reddit [1] but the poster never replied when people asked about the exact model and no one else reported this behavior, so I'm skeptical (I own a Samsung smart TV and from my experience it always asks you before attempting to connect to any wifi network). From the manufacturers' point of view it wouldn't make much sense auto-connecting to an arbitrary open network as that will in many cases just lead to an unusable connection, so I have a hard time believing Samsung would actually do this. What's more likely is that someone else (spouse, kid) used the TV, selected an open wifi and the TV saved that without OP knowing. But who knows, maybe Samsung is really after us.
Anyway, if you're so worried about the device maker spying on you it's probably best to avoid buying such a device (or any kind of smart device including a smartphone). In that case just use a monitor instead.
I cannot find it on my phone right now but I remember somebody catching their smart TV actually doing exactly that. Maybe somebody else has a link.