If you're one of those customers who has to do this, connect it via ethernet, do the setup, then forever leave it disconnected. Never give it your wireless credentials. I personally wouldn't trust the TV to forget the credentials and not phone home and make itself the ad machine the company wants it to be
If you don’t have this ability because the TV isn’t close to a router or if the manufacturers get so scummy that they no longer include Ethernet ports, you could accomplish this by setting up a “burner” hotspot on your phone and doing tethering. Change the name and password to something you’d never use and then let the TV connect to that and then change it to something else. TV may remember it but it won’t find that exact combo again unless you want it to :)
Because it's not so much brand specific as it's device specific. Companies are still testing the waters with adding this to their differently priced lineups to see what backlash they get from each group.