Gaining top position on the ground creates a significant advantage. It is very difficult to escape or retaliate from there, even with “dirty” tactics. In this instance, the person on the bottom cannot escape so attempts to bite, which only escalates the aggression of the other man who then beats him terribly.
Bystanders choose not to intervene, deciding that the beating is justified. If the man had not announced that the other had bitten him, they may have felt and acted differently. The opinion of bystanders is paramount and they need to be communicated with for this reason.