I'm just guessing out loud, but I think the M14S was originally intended for the international market where autos were allowed. Since today there is little point importing prohibs into Canada or Western Europe, and the US has an embargo on these Chinese Norc's entirely, there is not a lot of western market left. So the auto variant is pretty well a dead issue.
The M14S is selling like hotcakes though, so they may well have decided to stamp them S anyway to maintain the familiar M14 name. Basic marketing 101 to me...