There are no rules in a street fight. I don't agree. There are explicit rules-statutes regarding use of force and assault. There are implicit rules- each participant has a level of harm in which they are willing to inflict based on the situation and circumstance. The problem comes in when you don't know what your "opponent" will do. Will he fight by your schema?
I take a punch at you with the expectation that this will be a fist fight. In most cases I will be correct. Things may not work out for me if you pull a knife, or your friends decide to join in.
Break the explicit rules and you can get in trouble with the law and be penalized by the judge long after the "match" is over.
The implicit rules are an unknown. Is your opponent a reasonably well adjusted moral human being, a sociopath or something in between?
So a street fight is void of written rules or pre-arranged agreed upon rules but every participant has their own set of rules-that's what makes them dangerous.
Add alcohol, drugs or fear and those boundries can shift.