It's because tv and FM radio are in stereo. They are not "in Dolby Surround." Dolby Surround and PL simply need a regular stereo signal using the +,+ originally mentioned. Another way to say this is, it's a "playback" technology.Want to talk about deliberate?
Almost all of the TV programs I watch are in surround sound (using PL-II).
So are many of the commercials. Mostly the local commercials are in mono.
And I even hear a few surround commercials on FM radio (though most of the music is just stereo).
ALL movies are in stereo and thus "encode" the +,+ rear surround.
Yes, +,+ info can be manipulated for greater or lesser effect for playback on quad-type systems. (Which is what I think you meant) In other words, I hear all the effects too by amplifying the +,+ channel independently Eno style you hear with PL-II.
Not to beat a dead horse: there is no such thing as stereo that is not surround.