it's complicated, but to keep it short, it's part of the culture in North American hockey. It's really easy to intentionally injure people in hockey (you've got knives on your feet, clubs in your hand, and people can skate into each other at 25mph), so the rationale is that if you go headhunting a star player to take them out of the game, you will have to answer for it and fight a big scary guy who may not be the best player but he'll beat your ass to a pulp.

Refs are supposed to police the game, of course, but they aren't perfect and many transgressions are missed. Hence why players sometimes take matters into their own hands.

Fighting isn't allowed per se, but in most cases both players just have to spend the next five minutes in the penalty box. There are harsher rules for things like being the third man in in a fight, leaving the bench to fight, instigating a fight against someone who isn't willing to fight themselves etc.