Factual error: At the end of the first semifinal, as Cheng's gang gather to congratulate him, the events are being recorded for posterity by a photographer standing about 6 feet away, who, unfortunately is using at least a 200mm lens. I'm not sure he could even get Cheng's left eye into focus.
Factual error: When Dre and his mother leave Detroit for China, it is daytime, and the flight is 13 1/2 hours, then during the opening credits, night falls and then the sun comes up again. Since they were traveling west and time was going backwards through 12 time zones, there is no way it could have ever gotten dark. It would have been light the whole time.
Answer: Because Dre has finally earned Chengs respect. The other Fighting Dragons bowed to Mr. Han because in China bowing is shown as a sign of respect as they see that he is a better teacher and a kinder master then Master Li.
Since this movie mirrors the 1984 original movie, this is like how Johnny Lawrence (William Zabka) and the Cobra Kai's (excluding Dutch [Chad McQueen]) finally respect Daniel Larusso (Ralph Macchio), after the latter defeats Lawrence-The only difference is that the Cobras do not bow to Mr. Miyagi.