Back in the days when technical limitations forced developers to keep narratives short, side scrolling beat-em-ups required little justification for allowing combatants to batter each other senseless. Take Double Dragon for example: Boys see girl getting punched and kidnapped, boys punch everything in sight until they find girl again and then boys punch each other to see who wins the girl at the end. The boy and girl might have a fight off screen after the credit roll but then again that's just purely speculative. These were the good old days, the Wild West. The times where a vigilante approach was the only solution, causing people to unleash their own brand of violent justice onto the world.