Herb wasn't required to call off the fight because Bisping hit the ground. The bell rang a second later so Herb made the decision to let him keep going.
I've seen Herb stop fights for way less, but Bisping recovered on the stool so it is what it is.
Personally I think if the bell didn't ring a second later he would have waved it off and I think he was relieved the bell rang because he wanted to save face for the mouth piece.