After thinking about it, I believe it was the right call.
So if a fighter believes he wont win, he can just fall back, fake an injury and get a NC? Imagine if the other fighter was getting knocked around, then just falls on his elbow in round 3 and starts rolling around calling foul? That wouldnt be fair to call that a NC. He wanted a way out.
Not saying Hopkins wanted a way out, because he didn't.
Just saying that if no foul is called and the fighter cannot continue, it's a TKO.
If you think it should be a NC, then any fighter would just pull that move to avoid the L.