You missed the word "repeated". Also, some participants in drunken fights act in self-defense or as part of a fighting sport, and so would not be criminals.
So it's actually like this:
You reach 25. You assault somebody. You serve time in prison. (now a questionable case) We decide to give you a second chance. You feel no need to follow the law, and you enjoy assaulting people, so you do it again. OK, at this point we know what kind of person you are. You didn't just make a bad decision one day. Simply letting you out into society is pretty much the same as assaulting a random person, because that is clearly what you are going to do.
So it's actually like this:
You reach 25. You assault somebody. You serve time in prison. (now a questionable case) We decide to give you a second chance. You feel no need to follow the law, and you enjoy assaulting people, so you do it again. OK, at this point we know what kind of person you are. You didn't just make a bad decision one day. Simply letting you out into society is pretty much the same as assaulting a random person, because that is clearly what you are going to do.