Yeah, what #3 said....
Of course you would have to stock a bunch of different frequencies, since it isn't blocking FOX but the frequency it is being transmitted on, and that varies from locale to locale and cable system to cable system.
And only lazy, ignorant people who value symbolism over substance (in other words, your typical liberal) would buy the silly things. Even my little $60 cheapie TV has provisions for programming out any channel or channels you want.....
I suppose I could program out CNN CBS ABC NBC etc, but if I don't want to watch something, I just move on up or down the "dial".
The guy selling these things is following the old W.C. Fields adage: "Never give a sucker an even break".