None of us really have a clue what goes on behind closed doors at the club. Koeman might well have very good reasons for treating Niasse like he has. Maybe Koeman feels that players need to earn his respect.
I don't like bringing players personal lives into debates like you have with Gibson, I think it's very disrespectful (so it's ironic that you use that argument to defend Niasse and his treatment) and quite frankly none of our business unless it effects the club. However since you went there I'll say you picked the wrong man to defend in Niasse. Any man whose been charged with hitting his missus deserves zero respect in any walk of life as far as I'm concerned. Perhaps Koeman feels the same way and has acted accordingly.