Guest wifi does not mean it is unsecure, it is simply just another logical network. Sure amazon could equip their trucks with wifi I suppose and maybe some TVs would have good connection to update fast enough while a truck is there without a lot of tcp retransmits due to lack of efficient lack of penetration but that's not going effect all brands and surely it isn't something that is currently happening in a large effect.

You could talk about hypotheticals in the future sure but they aren't going to scan for these magical "network ports" that are just hanging around the ether. It needs to have a connection and one that is reasonable in quality and time.