1) Besides access control, having an unencrypted WiFi network is not great because your entire internet/local traffic is open to be sniffed. High-gain antennae, etc. You are essentially screaming the contents of (what would be) your ethernet cable into the void.

2) Access control is exactly the point. I suppose you could ban the TV's MAC address from your open network (and I suppose a sufficiently malicious TV firmware could randomize its MAC address), but your stated purpose (allowing open network access) is not exactly compatible with what you presumably wish to do (disallow network access to a smart TV).