system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why is hockey the only major non combat sport where fighting is allowed ?

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gpt
It isn't allowed per se. The players still get penalized. The severity of the penalty varies league by league; in minor leagues it is a game misconduct and the player is ejected, in the NHL it is a 5-minute penalty and the player is ineligible to play for the next 5 minutes. 

Fighting was banned in both basketball and baseball because they are both (mostly) non-contact sports. Football is a contact sport however there is just as much downtime as there is playing time therefor the tension and player temper is a lot lower. 

With the fast pace of hockey, tension and tempers can boil over. A lot of minor slashes, hits and bumps that would otherwise not be judge to be worth a penalty annoy the opposing player and sometimes they retaliate and a fight ensues. 

In the past, fighting was used as a deterrent to protect more skilled players. Dave Semenko is probably the most famous for this as his main roll was to protect Wayne Gretzky; if any opposing player dare touch Gretzky, he'd have to answer to Semenko. 

This led to other teams employing fighters as a strategy to take care of other teams tough guys. This was the era of the enforcer. Bob Probert, Tie Domi and many other players had very little role other than to fight the other teams tough guy. One common justification is that winning a fight could "swing momentum" in a game and your team would play better. This has not been proven despite study. 

In the mid 90s the NHL tried to curtail fighting by adding the "enforcer rule". If a player is judged to be the initiator of a fight that player may be penalized an extra 2 minutes and/or receive a 10-minute misconduct. That cut down on fights as retaliation. 

In short, the NHL allows fighting because they've never banned it. The keepers of the game view it as an important part of the sport.